An experiment investigated the effects of amplitude ratio (-35 to 35 dB in 10 dB steps) and fundamental frequency difference (0, 3, 6 and 12\%) on the identification of pairs of concurrent synthetic vowels. Vowels as weak as -25 dB relative to their competitor were easier to identify in the presence of a fundamental frequency difference (\deltafo). Vowels as weak as -35 dB were not. Identification was generally the same at \deltafo\ = 3\%, 6\% and 12 \% for all amplitude ratios: unfavorable amplitude ratios could not be compensated by larger \deltafo s. Data for each vowel pair and each amplitude ratio, at \deltafo\ = 0\%, were compared to the spectral envelope of the stimulus at the same ratio, in order to determine which spectral cues determined identification. This information was then used to interpret the pattern of improvement with \deltafo\ for each vowel pair, to better understand mechanisms of \fo-guided segregation. Identification of a vowel was possible in the presence of strong cues belonging to its competitor, as long as cues to its own formants F1 and F2 were prominent. \deltafo\ enhanced the prominence of a target vowel's cues, even when the spectrum of the target was everywhere up to 10 dB below that of its competitor. The results are incompatible with models of segregation based on harmonic enhancement, beats, or channel selection.