Purpose: To determine the specific acoustic changes that underlie improved vowel intelligibility in clear speech. Method: Seven acoustic metrics were measured for conversational and clear vowels produced by 12 talkers--6 who previously were found (S. H. Ferguson, 2004) to produce a large clear speech vowel intelligibility effect for listeners with normal hearing identifying vowels in background noise (the "big benefit talkers"), and 6 who produced no clear speech vowel intelligibility benefit (the "no benefit talkers"). Results: For vowel duration and for certain measures of the overall acoustic vowel space, the change from conversational to clear speech was significantly greater for big benefit talkers than for no benefit talkers. For measures of formant dynamics, in contrast, the clear speech effect was similar for the 2 groups. Conclusion: These results suggest that acoustic vowel space expansion and large vowel duration increases improve vowel intelligibility. In contrast, changing the dynamic characteristics of vowels seems not to contribute to improved clear speech vowel intelligibility. However, talker variability suggested that improved vowel intelligibility can be achieved using a variety of clear speech strategies, including some apparently not measured here. (Contains 2 tables, 5 figures and 1 footnote.)