The particle nucleation processes involved in the microemulsion polymerizations of styrene and methyl methacrylate have been investigated using pyrene fluorescence intensity measurements as a function of conversion. Very distinctive differences were observed between the results for the two different monomers. The styrene fluorescence results were consistent with nucleation occurring within the microemulsion droplets, followed by the subsequent heterocoagulation of the nucleated particles with remaining droplets. The very different methyl methacrylate data were consistent with a homogeneous nucleation mechanism.