Vanadium (V)-doped GaAs (GaAs:V) layers grown by metal-organic chemical vapour deposition under different V-doping levels (10(17)-10(19) cm(-3)) were annealed in an arsine-H-2 gas mixture up to annealing temperatures of 750 and 850 degrees C for 30 min. The effect of thermal treatments on their electrical and optical properties was studied by means of the Hall effect, deep level transient spectroscopy and photoluminescence (PL). Annealing at 750 degrees C induces a thermal conversion from the n- to p-type of weakly V-doped GaAs. The conductivity of highly V-doped materials remains n-type. All the V-doped samples convert from n- to p-type following annealing at 850 degrees C. A comparison between the PL spectra for materials annealed under different conditions underlines the important role of gallium vacancies and a possible V accumulation at the surface in the case of thermal conversion.