Donor compensation by associates of donor impurities and cation vacancies is investigated in vapor grown ZnSe: I and Al diffused melt grown bulk crystals. Both the (I-Se V-Zn) and the (Al-Zn V-Zn) A-centers could be identified by electron paramagnetic resonance (EPR) spectroscopy. The concentrations of the compensating defects can be reversibly changed by vapor-phase equilibration and a reproducible adjustment of room temperature (RT) carrier concentrations n(e) up to 1.4 x 10(18) cm(-3) (ZnSe : I) and 2.7 x 10(18) cm(-3) (ZnSe : Al) is possible. The results are explained in tenns of a defect chemical model. The supposed high concentrations of one type of vacancy associated defects are confirmed using positron annihilation (PA). Lowered compensation leads to an increase of carrier mobility mu(e) with increasing n(e). In low doped systems mobilities of 500 cm(2)/Vs can be obtained. A similarly drastic shift of the absorption edge to lower photon energies is observed for both I- and Al-doped crystals.