Subjects identified concurrent synthetic vowel pairs in four experiments. The first experiment found that improvements in vowel identification with a difference in fundamental frequency (DF0) do not depend on component phase. The second investigated more precisely whether phase patterns resulting from ongoing phase shifts in inharmonic stimuli can by themselves produce effects similar to those attributed to differences in harmonic state of component vowels. No such effects were found. The third experiment found that identification was better for harmonic than for inharmonic backgrounds, and that it did not depend on target harmonicity. The first three experiments employed a task in which subjects were free to report one or two vowels for each stimulus. The fourth experiment reproduced several conditions with a more classic task in which subjects had to report two vowels. Compared to the classic task, the new task gave larger effects and provided an additional measure of segregation: the number of vowels reported per stimulus. Overall, results were consistent with the hypothesis that the auditory system segregates targets by a mechanism of harmonic cancellation of competing vowels. They did not support the hypothesis of harmonic enhancement of targets. The lack of a phase effect places strong constraints on models that exploit pitch period asynchrony (PPA) or beats.