The synthesis of cadmium sulphide (CdS) semiconductor particles by precipitation and the influence of different synthesis parameters on the crystalline size of the particles are reported. The response surface methodology (RSM, Box-Behnken) was used to optimize the operating conditions in order to minimize the crystalline size. The values of the parameters studied were in the following range: pH value of the solution: 5 to 10; treatment time: 2-6 h; and mixing rate: 1.5 x 10(3) to 4.5 x 10(3) rpm. The optimal conditions for achieving the minimum crystalline size of CdS were as follows: pH = 7.26, mixing time = 2 h, and mixing rate = 4.5 x 10(3) rpm, and the minimum crystalline size achieved under these conditions was 6.9 nm.