Event-related potentials (ERPs) to different types of infrequent change in a tone pair composed of two closely spaced tones of different frequencies were recorded. The mismatch negativity (MMN), a change-specific component of the ERP, was elicited by reversing the order of the two tones, by repeating the first tone, by replacing the first tone with the second tone, or by omitting the second tone. The omission of the second tone, however, elicited the MMN only when the interval between the two tones was very short (offset to onset 40 or 140 ms) but did not when this interval was somewhat longer (240 or 340 ms). The pattern of the present results suggests that sensory-memory traces, as reflected by the MMN, integrate information about two closely spaced stimuli into a unitary sensory event.