My understanding is that a single "s" sounds more similar to the way that a "z" is pronounced in English, whereas the "ss" sound is more similar to the "s" sound of English.

Okey I will give you the Rule : 1- When the 'S' comes between two voyels , it is pronunced like the english ' Z '. Ex: RaiSin --> Z / poiSon ----> Z / maiSon --> Z 2- The double 'SS' is ALWAYS pronunced like the english stressed 'S' . Ex: MouSSe / poiSSon / miSSion... Hope it helps ! :)))

same thing for "dessert" (sssss) et "désert" (z).

Many students find that pronunciation is the hardest part of learning French. The new sounds, the silent letters, the liaisons... they all combine to make speaking French very tricky. So, the first question any thoughtful person might ask when learning French is, "French Pronunciation, is it possible?" A reasonable question. The letter S can be pronounced two different ways in French: It's usually pronounced like the English S, if not, it is pronounced like a Z The most important rule for S: when there is a double S betwen 2 syllables! POISSON - double S is placed betwen I and O However, single S place between 2 syllables becomes Z poison /with z/ Tricky pronunciation rules- The French R The French R has been the bane of French students since time immemorial. OK, maybe it's not quite that bad, but the French R is pretty tricky for a lot of French students. The French U The French U is another tricky sound, at least for English speakers, for two reasons: it's hard to say and it's sometimes difficult for untrained ears to distinguish it from the French OU. Nasal vowels are the ones that make it sound like the speaker's nose is stuffed up. In fact, nasal vowel sounds are created by pushing air through the nose and mouth, rather than just the mouth like you do for regular vowels. Good luck!