The perception of the vowel pair /ɑ‐æ/ was studied both in isolation and in the context /d‐dɔl/ (bottle‐battle). A set of eight stimuli were created on a speech synthesizer by varying second formant center frequency in equal logarithmic steps, such that Stimulus 1 sounded like / ɑ / or /bɑdɔl/ and Stimulus 8 sounded like /æ/ or /bædɔl/. The method of absolute identification was used to analyze the psychological distance between adjacent stimuli in terms of d′. It was found that the phoneme boundary effect was observed for the vowel pair in word context only; subjects could differentiate between stimuli in the center of the continuum (stimulus Pairs 4 and 5) much easier than between stimuli at the ends of the continuum (Pairs 1 and 2 or 7 and 8). Isolated vowel stimulus pairs (approximately 250 msec in duration) were differentiated equally well over the whole continuum. [This work was supported in part by the National Institutes of Health.]