Two experiments investigated the effects of small values of fundamental frequency difference (ΔF0) on the identification of concurrent vowels. As ΔF0’s get smaller, mechanisms that exploit them must necessarily fail, and the pattern of breakdown may tell which mechanisms are used by the auditory system. Small ΔF0’s also present a methodological difficulty. If the stimulus is shorter than the beat period, its spectrum depends on which part of the beat pattern is sampled. A different starting phase might produce a different experimental outcome, and the experiment may lack generality. The first experiment explored the effects of ΔF0’s as small as 0.4%. The smallest ΔF0 conditions were synthesized with several starting phases obtained by gating successive segments of the beat pattern. An improvement in identification was demonstrated for ΔF0’s as small as 0.4% for all segments. Differences between segments (or starting phase) were also observed, but when averaged over vowel pairs they were of small magnitude compared to ΔF0 effects. The nature of ΔF0-induced waveform interactions and the factors that affect them are discussed in detail in a tutorial section, and the hypothesis that the improvement in identification is the result of such interactions (beat hypothesis) is examined. It is unlikely that this hypothesis can account for the effects observed. The reduced benefit of ΔF0 for identification at smaller ΔF0’s more likely reflects the breakdown of the same F0-guided segregation mechanism that operates at larger ΔF0’s.