Theoretical calculations have been made for the nucleation kinetics during the growth of ZnSxSe(1-x) single crystals by chemical vapour transport technique using iodine transporting agent. The partial pressures of S-2, Se-2, ZnI2, I and I-2 were calculated using a thermodynamic model at different deposition temperatures (800-900 degreesC) using different iodine concentrations (0.5-10 mg cm(-3)). Supersaturation ratios of S-2 and Se-2 which affect the formation and composition of ZnSxSe(1-x) single crystals during growth have been calculated. The size and free energy of formation of a critical nucleus have been calculated using classical nucleation theory as a function of growth temperature and iodine concentration. The growth of ZnSxSe(1-x) single crystals has been carried out by CVT method using iodine transporting agent. The XRD and etching studies have been carried out to the grown crystals. The results were correlated with the theoretical calculations. (C) 2004 Elsevier B.V. All rights reserved.