A windowing polarization technique has been applied to form PMMA thermoelectrets. The stimulated depolarization currents of these electrets in the range of polarization temperatures between 380 and 405 K show only a rho peak which is attributed to space charge relaxation. The area of this peak exhibits a maximum at a certain polarization temperature (the optimal polarization temperature). The results obtained by fitting the depolarization currents to the general kinetic order model are consistent with the existence of a space charge polarization mechanism with a distribution of trapping depths, which can be qualitatively described. Moreover, the Arrhenius-like kinetic parameters deduced from the relaxation times can be interpreted in terms of a compensation law. Finally, the optimal polarization in PMMA can be described by means of a distribution of trapping levels.