The perception of vowel quality is not only affected by the formant structure of the vowel, but by the nature of the surrounding phonetic context. For example, it has been demonstrated [R. A. Fox, in Speech Perception, Production and Linguistic Structure, edited by Y. Tohkura et al. (IOS Press, 1992)] that listeners' ability to make reliable vowel quality distinctions are lowered when the vowels are placed in a context that introduces phonological neutralization (e.g., post-vocalic [(inverted are)]). One possible explanation for this phenomenon is that of listener bias, which should be eliminated using the appropriate signal detection methodology. The present study describes the results of an experiment that obtained d' measurements of one-step differences in an [i]-[(inverted are)] continuum in CV, CVr, CVl, and CVd contexts under fixed-level, minimum uncertainty conditions. Results demonstrate after several days experience with the stimuli and methodology that the discrimination differences as a function of context were eliminated. However, a post-training identification test still showed context effects on vowel categorization. These data will be discussed in terms of the possible nature of this listener bias in vowel perception.