In order to assess the interaction of spectral and durational differences that may affect their relative importance as cues for vowel contrasts, two concept formation experiments were conducted in which subjects were trained to categorize synthesized vowel stimuli that differed in the value of Fl and in duration. In Experiment 1, Fl of the two category tokens differed by 44 Hz (0.4 bark) and in Experiment 2, by 131 Hz (1.2 bark). In both experiments the duration difference of the two stimuli were the same (10 ms). After subjects had learned to distinguish the stimuli, they were asked to categorize new stimuli. How the subjects responded to these new stimuli was used to evaluate the relative salience of durational and spectral differences. Although the duration differences were the same in both studies, the results in Experiment 1 indicated that duration was the more salient property for the category distinction, while in Experiment 2, the Fl difference was not only more salient but effectively masked the duration difference. These results strongly suggest that whether or not an acoustic parameter is a cue for a category distinction will depend on its strength relative to other co-occurring differences.