The phonetic quality of front vowels along a synthetic /I/‐/e/ FI continuum can be influenced by passing the vowels through a filter that has a sloping characteristic in the FI region. The shift in phoneme boundary that occurs for different filter slopes was measured and whether the influence of various conditions listeners can compensate for the characteristics of the transmission channel was examined. These conditions include placing the vowel in a consonantal context and preceding the vowel with a carrier phrase that has been subjected to the same filtering as the vowel. Experiments have also been performed on the preception of natural vowels in isolation and in a consonantal context. [Work supported by SERC.]