This paper describes the development of vowel inventories in 12 pre-linguistically deafened Cantonese-speaking cochlear implant users. The aim of the research was to understand the role of linguistic, perceptual and articulatory factors in determining the rate and order of vowel acquisition in this population of children. Rate and order of vowel acquisition were reported against two criteria. The "targetless" criterion for production required at least two correct productions of a vowel in a sample of speech. The second or "target" criterion required that a vowel first met the targetless criterion and was further produced correctly in the appropriate phonological context at least 50% of the time. Factors associated with articulatory difficulty were shown to explain the composition of pre-implant vowel inventories. Subsequent to receiving an implant, all children demonstrated a steady and systematic expansion in the size of their vowel inventories though the rate of acquisition varied between individuals. The order of vowel acquisition was affected by a combination of linguistic and articulatory factors. The paper concludes by discussing the clinical implications of these findings for helping children develop good spoken language skills subsequent to receiving an implant.