We have found that undoped semi‐insulating GaAs substrates convert from semi‐insulating to strongly conductingp‐type behavior following high‐temperature annealing (830 and 885 °C) for long periods of time (22–60 h). Remarkably similar results have been obtained for both liquid‐encapsulated Czochralski and horizontal Bridgman undoped semi‐insulating GaAs synthesized by various manufacturers. Hole concentrations measured at room temperature are aboutp=9.1×1015and 3.6×1016cm−3after 830 and 885 °C anneals, andpis uniform throughout the 380 to 510 &mgr;m substrate thickness for 885 °C anneals. Hall‐effect measurements of carrier freeze‐out indicate that residual carbon acceptors are the dominant shallow‐acceptor species. Conversion toptype appears to arise from out‐diffusion of EL2 deep donors and from formation of an additional acceptor‐type native defect which also results from arsenic out‐diffusion during annealing.