A combined study of beta' and alpha relaxations in poly(methyl methacrylate) (PMMA) by thermally stimulated depolarization currents and thermal step methods has been made. The use of null width polarization windows to form the electrets allows us to isolate the beta' peak. The charge distribution profiles allow us to assign a polar orientational mechanism for the Pl relaxation and in the case of the alpha peak a background polar mechanism over which a temperature-dependent injection and a conduction processes are overlapped. The charge distribution profiles observed by the thermal step method indicate that the increase of the conductivity observed in this material at temperatures just below the glass transition is mainly due to the positive carriers.