Acoustic and perceptual effects of ``clear speech'' were investigated by focusing on two duration-dependent vowel contrasts in American English, /(cursive beta), (ae ligature)/ and /(inverted vee), (inverted aye)/. Minimal pairs of words were recorded in carrier sentences by four native female speakers under two speech style conditions, conversational and clear. Measurements of vowel duration and formant frequency were made. The acoustic analyses revealed patterns of durational behavior that differed by speaker but were generally uniform across both vowel contrasts. F1 and F2 frequencies from the grouped data were displaced toward more peripheral regions of the vowel space in clear speech. However, no consistent patterning was evident across individual speakers. A perceptual test using gated speech stimuli from two of the speakers was used to assess the relative amount of acoustic signal required to identify each vowel per speech style. Also, vowel identification rates were examined for initial gated segments (which included the first 25 ms of the vowel) as well as for full word presentation. Statistical analyses showed significant effects of speech style on vowel identification. Interactions among several factors, however, indicate that the perceptual improvements from speaking clearly depend on the speaker, vowel category and subsequent consonantal context.