It depends on accent, and short vowels tend to be pure in all accents of English, to the extent they are in other languages. Long vowels can become diphthongs

It depends on accent, and short vowels tend to be pure in all accents of English, to the extent they are in other languages. Long vowels can become diphthongs though. Here are the vowels in Received Pronunciation and General American (and one other accent): https://en.wikipedia.org/wiki/Lexical_set https://en.wikipedia.org/wiki/Lexical_set Even RP has some pure long vowels (e.g. NURSE, THOUGHT, BATH). FLEECE and GOOSE do tend to be realized as narrow diphthongs, though. In my own accent (Scottish), except for the four diphthongs (PRICE, PRIZE, CHOICE, and MOUTH) of which the first two are allophones, all the other vowels are pure. I verified this while developing a formant speech synthesizer.