Solubilities and rates of substitutional diffusion of aluminium and gallium in polycrystalline zinc oxide have been determined in the temperature range 750-1000� by a chemical method previously described. The solubility of aluminium in zinc oxide, expressed in ions cm-3, is given by the expression n = 1.0 x 1023exp(-1.08k-1T-1), and the solubility of gallium by n = 2.7 x 1021exp(-0.59k-1T-1), where the activation energies are expressed in eV. The rates of diffusion of both aluminium and gallium were found to be dependent on surface concentration up to a limiting value, at which the diffusion coefficient of aluminium is D = 5.3 x 10-2 exp(-2.74k-1T-1) cm2 sec-1, and that of gallium is D = 3.6 x 104 exp(-3.75k-1T-1) cm2 sac-1. ��� It is shown that supersaturation of both aluminium and gallium in the zinc oxide crystals occurs above 800�.