Temporal synchrony is a critical condition for integrating information presented in different sensory modalities. To gain insight into the mechanism underlying synchrony perception of audio-visual signals we examined temporal limits for human participants to detect synchronous audio-visual stimuli. Specifically, we measured the percentage correctness of synchrony-asynchrony discrimination as a function of audio-visual lag while changing the temporal frequency and/or modulation waveforms. Audio-visual stimuli were a luminance-modulated Gaussian blob and amplitude-modulated white noise. The results indicated that synchrony-asynchrony discrimination became nearly impossible for periodic pulse trains at temporal frequencies higher than 4 Hz, even when the lag was large enough for discrimination with single pulses (Experiment 1). This temporal limitation cannot be ascribed to peripheral low-pass filters in either vision or audition (Experiment 2), which suggests that the temporal limit reflects a property of a more central mechanism located at or before cross-modal signal comparison. We also found that the functional behaviour of this central mechanism could not be approximated by a linear low-pass filter (Experiment 3). These results are consistent with a hypothesis that the perception of audio-visual synchrony is based on comparison of salient temporal features individuated from within-modal signal streams.