A GaP(0 0 1) surface treated by S2Cl2 and P2S5/(NH4)(2)S-x, solutions has been investigated by AES and XPS. The amount of sulfur on the surface treated by the P2S5/(NH4)(2)S-x, solution is more than that by the (NH4)(2)S-x. The sulfur coverage on the S2Cl2- and P2S5/(NH4)(2)S-x-treated samples annealed at 550 degreesC was estimated to be about 1.2-1.3 monolayers. It is found that the adsorbed sulfur is stable below 550 degreesC and oxygen on the surface is almost removed at 550 degreesC. Chlorine atoms on the GaP(0 0 1) surface treated by S2Cl2 were easily desorbed upon annealing. The XPS result indicates that the adsorbed sulfur is bonded mainly to the Ga atoms on the surface. (C) 2004 Elsevier B.V. All rights reserved.