Positron lifetime measurements have been carried out in n-type Sn-doped GaAs (n similar to 5 x 10(17) cm(-3)) during isochronal annealing over a temperature range 20-520degreesC. The annealing results have revealed a significant change in positron lifetime parameters beyond 320degreesC indicating a configurational change in the acceptor type vacancy-dopant complex in the material. The long lifetime component 295ps obtained at room temperature has been attributed to this specific acceptor type vacancy-dopant complex.