A theoretical model for optical spin pumping of electrons in a quantum well with low intrinsic electron density is presented. A system of electrons under continuous-wave illumination by circularly polarized light tuned to the electron-trion resonance is considered. The simultaneous off-resonant creation of excitons is also taken into account. The spin flip of trions and their radiative decay as the basic processes which allow the electronic spin pumping, as well as other processes, such as the formation of trions from excitons and electrons, are accounted for in the appropriate kinetic equations. The results obtained for CdTe and GaAs quantum wells indicate that significant electron spin polarization can be achieved in a time range of a few nanoseconds.