The single letter <s> can sometimes represent the sound ʒ. Here are some examples of this.
When preceded by a vowel and followed by the ending <ure>, as in
closure leisure measure pleasure treasure
If a consonant precedes the <s> then the pronunciation is ʃ, as in censure.
In the word endings –asion, –esion, –ision, –osion, –usion, as in
invasion lesion decision erosion inclusion
For some speakers, in the words Asia and Asian (and compounds such as Australasian). Other speakers have ʃ in these words.
casual usual usually visual
Again we have ʃ if a consonant precedes, as in sensual.