The influence of spectral cues on discrimination peaks in the region of the phonetic voicing boundary was examined. The discriminability of voice onset time (VOT) differences of the same temporal magnitude was assessed using stimuli from labial and velar consonant–vowel VOT continua that differed in the timing of spectral changes associated with the first formant (F1) transition, and in the location of the phonetic boundary. Subjects were initially given labeling tests and fixed‐standard AX and all‐step discrimination tests on both series. Half the subjects then received all‐step discrimination training on one series and half received training on the other series. Finally, all subjects were again given the labeling and discrimination tests on both series. Just noticeable differences (jnds) in VOT were estimated from the all‐step functions before and after training. Initial jnds showed that VOT discrimination was most accurate around the voicing boundary on the two continua, where differences inF1 onset frequency accompany variations in VOT. jnds on both series decreased significantly after training, although these regions of greater sensitivity remained. No evidence was seen of increased sensitivity around ±20‐ms VOT, as expected if auditory processing constraints were influencing temporal order judgments. Comparisons of post‐training jnds within and across series indicated that spectral components of VOT, primarilyF1 onset frequency differences, exert a substantial influence on discrimination, and, along with other spectral cues provided by source differences at stimulus onset, can account for the discontinuities in discrimination often reported in research with VOT continua. Large phonetic effects also were seen in the initial performance of all subjects: jnds decreased consistently as standards drew nearer the voicing boundary. However, these effects were absent in the final jnds for most subjects. Implications of these findings for the understanding of basic auditory and attentional processes in speech perception are discussed.