Infrequent stimuli deviating qualitatively from a train of standard stimuli appear to evoke two 'N2' potentials: A modality-specific 'mismatch negativity' (N2a), and a sharp peak forming part of a biphasic vertex response (N2b/P3a). Previous investigations leave it uncertain whether the same pattern of potentials is elicited by temporal deviance. In the present experiment a chequerboard was flashed at a standard ISI of 2000 msec, with occasional flashes occurring early at ISIs of 500, 1000 or 1500 msec. Detected deviants evoked N2b/P3a at an amplitude which increased with degree of deviance, but it was also evoked, at lower amplitude, by missed deviants and by standards. Its amplitude seems to depend upon the discriminability of the deviant stimuli, rather than upon whether they are actually detected. Moreover deviance from a repetitive background does not appear to be a necessary condition, since the complex can be elicited by isolated stimuli presented at irregular intervals. These considerations suggest that temporal uncertainty is an important factor. Temporal deviance produced no discernible mismatch negativity, modality-specific or otherwise, suggesting that it may involve processes different from those engaged in qualitative deviance.