We show that the electrical compensation of undoped GaAs grown by the liquid encapsulated Czochralski technique is controlled by the melt stoichiometry. The concentration of the deep donor EL2 in the crystal depends on the As concentration in the melt, increasing from about 5×1015cm−3to 1.7×1016cm−3as the As atom fraction increases from 0.48 to 0.51. Furthermore, we show that the free‐carrier concentration of semi‐insulating GaAs is determined by the relative concentrations of EL2 and carbon acceptors. As a result, semi‐insulating material can be obtained only above a critical As concentration (0.475‐atom fraction in our material) where the concentration of EL2 is sufficient to compensate residual acceptors. Below the critical As concentration the material isptype due to excess acceptors.