Electron irradiation as an alternative to chemical initiation in emulsion polymerization is a well-known but not widely used technique. Accelerated electrons create the initiating radical species by scission of molecules that are present in the reaction mixture. The initiation mechanism in detail is so far not completely understood due to the broad variety of radical species that are created by the electron beam. The pulsed electron beam polymerization (PEBP) of methyl methacrylate (MMA), ethyl methacrylate (EMA), and butyl methacrylate (BMA) was analyzed with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to shine light onto the main initiating species of the PEBP but also on the polymer decomposition that occurred under pulsed electron beam irradiation. Hydrogen and hydroxyl radicals were identified as the main initiating species in polymerization of the investigated monomers. An increasing degree of initiation by monomer radicals was observed with decreasing water solubility of the polymerized monomer.