Some languages, such as many varieties of English, use short-lag and long-lag VOT to distinguish word- and syllable-initial voiced vs. voiceless stop phonemes. According to a popular view, the optimal VOT category boundary between the two types of stops moves towards larger values as articulation rate becomes slower (and speech segments longer), and listeners accordingly shift the perceptual VOT category boundary. According to an alternative view, listeners do not shift the VOT category boundary with a change in articulation rate, because the same category boundary remains optimal across different rates of articulation in normal speech, although a shift in the optimal boundary location can be induced in the laboratory by instructing speakers to use artificially extreme articulation rates. In this study we compared the effectiveness of rate-independent VOT category boundaries applied to word-initial stop phonemes in spontaneous English speech, against the effectiveness of Miller et al.’s ( 1986) rate-dependent VOT category boundary applied to laboratory speech. The effectiveness of the two types of category boundaries were comparable, when spontaneous speech data were controlled for factors other than articulation rate. Our results suggest that perceptual VOT category boundaries need not shift with a change in articulation rate under normal circumstances.