Two experiments investigated the effects of small values of fundamental frequency difference (\deltafo) on the identification of concurrent vowels. As \deltafo s get smaller, mechanisms that exploit them must necessarily fail, and the pattern of breakdown may tell us which mechanisms are used by the auditory system. Small \deltafo 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 \deltafos\ as small as 0.4\%. The smallest \deltafo\ conditions were synthesized with several starting phases obtained by gating successive segments of the beat pattern. An improvement in identification was demonstrated for \deltafos\ 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 \deltafo\ effects. The nature of \deltafo-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 \deltafo\ for identification at smaller \deltafo s more likely reflects the breakdown of the same \fo-guided segregation mechanism that operates at larger \deltafo s.