The compensation degree of GaAs : Si crystals grown by horizontal Bridgman method was investigated as a function of As pressure during the growth. Three different methods (local vibrational mode (LVM) absorption, infrared absorption, Hall-effect and conductivity) were used for the determination of the compensation degree, K. These methods yield the same result of a minimum of K at a small excess of the Ga mole fraction in the melt. Possible compensation mechanisms are discussed to explain this minimum.