This study investigates the influence of the acoustic properties of vowels on 6- and 10-month-old infants’ speech preferences. The shape of the contour (bell or monotonic) and the duration (normal or stretched) of vowels were manipulated in words containing the vowels /i/ and /u/, and presented to infants using a two-choice preference procedure. Experiment 1 examined contour shape: infants heard either normal-duration bell-shaped and monotonic contours, or the same two contours with stretched duration. The results show that 6-month-olds preferred bell to monotonic contours, whereas 10-month-olds preferred monotonic to bell contours. In Experiment 2, infants heard either normal-duration and stretched bell contours, or normal-duration and stretched monotonic contours. As in Experiment 1, infants showed age-specific preferences, with 6-month-olds preferring stretched vowels, and 10-month-olds preferring normal-duration vowels. Infants’ attention to the acoustic qualities of vowels, and to speech in general, undergoes a dramatic transformation in the final months of the first year, a transformation that aligns with the emergence of other developmental milestones in speech perception.