The Two Sounds of the Letter S in the Italian Language

S as S | S as Z | The Rules

The letter S has two sounds in Italian. At first, when you study Italian, you don’t really notice it, but then after awhile you catch yourself pausing to stop and think about it. In short, S in Italian can sound like S in English or like Z in English. And as you would expect there are rules guiding pronunciation, and there are exceptions to those rules.

S as S

Italian words where the letter S is pronounced like an English S include:

sano, scatola, squardrare, disfare, rispetto, cassa, sasso, rosso, grosso, passo, pasto, abside, facsimile, falso, penso, Pepsi, orso, gas, borsa, denso, falso, sera, sale, cisterna, sfogliare, soggetto

This sound is referred to technically as a voiceless alveolar fricative (fricativa alveolare sorda in Italian) which indicates where the sound is made in the mouth. Fricatives are types of consonant sounds produced by forcing air through narrow channel. In this case, with the tip or blade of the tongue against the gum line just behind the teeth. The sound is denoted as /s/ in the International Phonetic Alphabet (IPA).

S as Z

Italian words where the letter S is pronounced like (or close to) an English Z include:

chiesa, casa, sgabello, sbaglio, sbavare, presentare, asma, scherzoso, tesi, crisi, battesimo, trentesimo, bisogno, esame, isola, slogan, smalto, snello, alpinismo, risvolto, esempio

This sound is referred to technically as a voiced alveolar fricative (fricative alveolare sonora in Italian). This sound is produced by “channeling air flow along a groove in the back of the tongue up to the place of articulation, at which point it is focused against the sharp edge of the nearly clenched teeth, causing high-frequency turbulence.” This sound is denoted as /z/ in the IPA.

A word showing both S sounds is scherzoso meaning "playful or humorous", which has - in order - one /s/ sound, one /ts/ sound (a true Z), and one /z/ sound. In IPA notation the pronunciation is /sker'tsozo/.

The Rules

These rules are a combination of rules given in the reference Grammatica italiana di base by Trifone and Palerma and those listed on the Italian Wikipedia page S sorda.

Pronounced /s/

• Words that start with an S followed by a vowel. 
• Examples: sera, sale, signora, soggetto, sei.
• Words where the letter S is followed by unvoiced consonant. 
• Examples: scatola, squadrare, disfare, rispetto, stentato, spendere, cisterna. 
• Unvoiced consonants are /k/ (hard c or q), /f/, /p/, and /t/. 
• Words where the letter S is preceded by any consonant. 
• Examples: abside, facsimile, falso, penso, transitivo, Pepsi, orso. 
• Words that end in S.
• Examples: gas, ultras, iris.
• But not always, note pronunciation of relais.
• Words with SS. 
• Examples: grosso, rosso, passo, fisso, commesso, asso.

Pronounced /z/

• Words where the letter S is followed by a voiced consonant.
• Examples: sbadigliare, sdoganare, sgrossare, alpinismo, snello, risvolto, slogan, sgabello. 
• Voiced consonants are /b/, /d/, /g/ (or /dz/), /l/, /m/, /n/, and /v/ 
• Technical words that end in -asi, -esi, -isi, or -osi.
• Examples: protesi, tesi, crisi, sclerosi, enclisi. 
• Words ending in -esimo and -esima.
• Examples: battesimo, umanesimo, trentesimo, cresima, quaresima. 
• Words where S is preceded and followed by a vowel (S between two vowels) are likely pronounced as /z/ but not always.
• Examples: bisogno, esame, isola, chiesa, sole. 
• For examples of the numerous exceptions, see below.

Exceptions – pronounced /s/ not /z/

• Some enclitic (joined) words. 
• Examples: affittasi, dicesi, qualsiasi, vendesi. 
• Tangential note:  affittasi appartamento is equivalent to appartamento è affittato. 
• Words that are formed from another word with a prefix added, or are otherwise composed.
• Examples: antisovietico, dinosauro, presidente, risultare, presentire. 
• Interesting case: ti presento – "I'll introduce you" – is pronounced with /z/ for the first person singular conjugation of the verb presentare. Compare that with  presento un pericolo – "I foresee danger" – pronounced with /s/ for the first person singular conjugation of the verb presentire, a verb composed of the prefix pre and the verb sentire.
• Words that have an alternate version with a doubled S. 
• Examples: musulmano (mussulmano), Albisola (Albissola) 
• The endings of some irregular verbs conjugated in the passato remoto and in their past participles.
• Examples: rendere (resi, rese, resero), scendere (scesi, scese, scesero), nascondere (nascosi, nascose, nascosero). 
• Some common words and their derivatives that have an S preceded and followed by a vowel (S between two vowels) that are either always pronounced as /s/ or can be /s/ or /z/ depending on where the speaker is from. 
• The sources consulted (given above) plus https://www.dizionario-italiano.it don’t always agree. Below, we’ll use the dizionario-italiano.it as our arbiter here. 
• Pronounced with /s/: spesa, chiuso, così, naso, peso, riso, risotto
• Pronounced with /s/ or /z/: casa, cosa, mese, Pisa, raso, posa, posare
• Derivatives of words in this category are subject to the same pronunciation.
• Examples: socchiusi, sorriso, risotto 
• As a rule of thumb, the words that can be either pronounced with /s/ or /z/ tend toward /z/ in northern Italy. (Don’t ask an Italian where northern Italy starts. Various answers we’ve heard are the Bolzano, the Po River, Florence, and Rome.)