Purpose: The effect of onset asynchrony on dichotic vowel segregation and identification in normal-hearing (NH) and hearing-impaired (HI) listeners was examined. We hypothesized that fusion would decrease and identification performance would improve with increasing onset asynchrony. Additionally, we hypothesized that HI listeners would gain more benefit from onset asynchrony. Method: A total of 18 adult subjects (nine NH, nine HI) participated. Testing included dichotic presentation of synthetic vowels, /i/, /u/, /a/, and /ae/. Vowel pairs were presented with the same or different fundamental frequency (f[subscript o]; f[subscript o] = 106.9,151.2, or 201.8 Hz) across the two ears and one onset asynchrony of 0, 1, 2, 4,10, or 20 ms throughout a block (one block = 80 runs). Subjects identified the one or two vowels that they perceived on a touchscreen. Subjects were not informed that two vowels were always presented or that there was onset asynchrony. Results: The effect of onset asynchrony on fusion and vowel identification was greatest in both groups when [delta]f[subscript o] = 0 Hz. Mean fusion scores across increasing onset asynchronies differed significantly between the two groups with HI listeners exhibiting less fusion across pooled [delta]f[subscript o]. There was no significant difference with identification performance. Conclusions: As onset asynchrony increased, dichotic vowel fusion decreased and identification performance improved. Onset asynchrony exerted a greater effect on fusion and identification of vowels when [delta]f[subscript o] = 0, especially in HI listeners. Therefore, the temporal cue promotes segregation in both groups of listeners, especially in HI listeners when the f[subscript o] cue was unavailable.