We examined phonetic and sensory processes in speech perception using mismatch negativity, an event-related potential component congruent with discrimination, but which occurs for unattended stimuli. Adult listeners (N=16) heard a repeated standard (the syllable 'da') that was interrupted infrequently by a phonetically different 'deviant' syllable ('ba'). The acoustic difference between standard and deviant was manipulated to create both acoustically Strong and Weak deviant stimuli. Mismatch negativities in response to the Strong deviant were significantly greater than those for the Weak deviant, in spite of the fact that both represented stable instances of the phonetic category. The data suggest that the mismatch negativity component can be strongly influenced by sensory factors beyond what is predicted by overt categorization and discrimination judgments.