The diffusion of beryllium and zinc into GaAs substrates during liquid phase epitaxial growth ofp‐Ga0.2Al0.8As layers was investigated. The diffusion coefficients were determined from the profile of free‐carrier concentration in thep‐GaAs region. Comparing the diffusion coefficients of Be and Zn, the diffusion coefficient of Be was found to be about ten times higher than that of Zn in the temperature range above 750 °C. For Be diffusion, if the surface free‐carrier concentration in thep‐GaAs layer is lower than about 6×1018cm−3, the diffusion can be expressed as diffusion from a semi‐infinite source and the free‐carrier concentration profiles are represented by the error function complements. With regard to temperature dependence, the diffusion coefficient takes the formD=D0 exp(−E0/kT), whereD0=0.66 cm2 s−1andE0=2.43 eV in the temperature range 700–900 °C. If the surface free‐carrier concentration was higher than about 6×1018cm−3, anomalous behavior was observed; i.e., the diffusion could be no longer represented by a constant diffusion coefficient.