The position of the phoneme boundary on an acoustical continuum between two CV test syllables can be shifted by prior repetition of an adapting CV syllable. Our experiments show that repeated exposure to a distinct /ba/ or /da/ adapter whose place of articulation is cued only by transitions of the third formant, with a fixed, ambiguous value of the second formant, does not shift the phonome boundary on a /ba/-/da/ test series if the test stimuli have no third formant and are cued by transitions of the second formant. However, the same adapters will shift the phoneme boundary of an otherwise similar test series which has a neutral transitionless fixed third formant. We interpret this result as indicating that (i) transitions of the second and third formants are processed independently at peripheral levels, and (ii) a substantial proportion of the place adaptation effect in CV syllables distinguished by characteristics of the formant transitions is at the level of detectors for acoustical cues, not at the level of categorical perceptual decisions or response organization. This does not deny the existence of other types and dimensions of adaptation effect.