We present a method that we developed to discern where the optical microvariability (OM) in quasars originates: in the accretion disk (related to thermal processes) or in the jet (related to non-thermal processes). Analyzing nearly simultaneous observations in three different optical bands of continuum emission, we are able to determine the origin of several isolated OM events. In particular, our method indicates that from nine events reported by Ramírez et al., three of them are consistent with a thermal origin, three with non-thermal, and three cannot be discerned. The implications for the emission models of OM are briefly discussed.