The solubility, diffusion, and electrical behavior of Li in GaAs have been investigated by means of chemical analysis, conductivity, and Hall‐effect measurements. Both floating‐zone and Bridgman crystals have been examined.The solubility of Li in GaAs from the equilibrium surface alloys has been determined from 500° to 1150°C. A maximum solubility of 4×1019Li cm−3occurs at about 1050°C. Solubilities from surface Li2O and gaseous LiBr have also been determined over limited temperature ranges.Li diffusion has been followed by electrical and chemical methods. The diffusion is nonideal and follows the relation,D*=0.53 exp(−1.0/kT), whereD* is the apparent diffusion coefficient. Li produces ``automatic'' compensation of GaAs. Out‐diffusion of Li results in acceptor formation. This behavior has been examined in detail and interpreted by postulating initial formation of stable (Li+Li=) doublets which, through the hole‐electron equilibrium, cause Li to dissolve as Li+donors to near compensation.A marked effect of donor doping on the solubility of Li has been found.Certain unexplained differences between crystals exist which are being further investigated.