Green and Dai [ inAuditory Physiology and Perception, edited by Y. Cazals, L. Demany, and K. Horner (Pergamon, Oxford, 1992)] reported a series of experiments which suggested that listeners’ ability to perform simultaneous across-frequency comparisons of intensity is severely impaired when the target and flanking components begin or end at different times. The present experiment sought to replicate the effect of onset asynchrony and included an additional condition in which the leading portion of the asynchronous target component was accompanied by a pair of “captor” tones. The intended purpose of the captor tones was to promote a perceptual organization in which the leading and synchronous portions of the asynchronous target component were grouped with different auditory objects. For all six listeners an asynchrony of 320 ms raised thresholds substantially relative to the synchronous onset condition, the magnitude of the increase ranging between 6 and 16 dB. By contrast, the mean elevation of threshold in the presence of the captor was only 3 dB, although for all listeners thresholds were still greater than for the synchronous onset condition. The results support the view that the deleterious effect of onset asynchrony on profile analysis performance is due to the operation of auditory grouping principles.