Abstract In electroencephalography (EEG), action execution (AE) is reliably associated with reductions in the mu rhythm, a periodic oscillation (8–14 Hz) over sensorimotor cortex. Similar patterns of “mu suppression” have been reported during observations of others’ actions, leading to claims that the mu rhythm indexes motor contributions to social perception. However, evidence for mu suppression during action observation (AO) is mixed, possibly due to methodological considerations (e.g. sample size, recording sites) and perceptual and attentional confounds. Moreover, measurements of periodic oscillations may be conflated with underlying aperiodic (“1/f-like”) neural activity, potentially influencing estimation of the EEG power spectrum. Here we examined the influence of aperiodic factors on mu suppression using 128-channel EEG in a large sample (N = 109), both during AE and in an AO task with appropriate visual and attentional controls. Whereas AE was consistently associated with significant mu suppression, we initially failed to find significant mu suppression for AO, suggesting that attentional and perceptual confounds may bias mu estimation during AO. However, removing the aperiodic component restored mu suppression over central electrodes. Although significant, mu suppression for AO appears less robust than originally reported, with aperiodic activity contributing to variability in the estimation of mu suppression during AO.