In a semiconductor in which the effect of mixed conduction is dominant, the determination of carrier mobilities only by Hall measurements is impossible. Magnetoresistance measurements provide the lacking equation which, in conjunction with the expressions forRHand &mgr;nin the mixed conduction region, may be used to determine &mgr;nandb. As the analytical solution is difficult, we solved the system by an approximative method under the main assumption thatb≫1. This method was applied to measurements of semi‐insulating Cr‐doped GaAs (which is a typical example of a semiconductor in which mixed conduction is dominant), with good results. Further application to intrinsic InSb at room temperature, using known data, confirmed the validity of the method.