The method of growing ZnS crystals in a closed tube containing HCl as a vapor solvent and located in a temperature gradient around 1000°C is investigated. Experiments show that net flux of solid is directly proportional to the cross sectional area of the container and practically unaffected by surface reaction rates. The flux of solid increases with the square root of the pressure up to about 2 atm and is thereafter almost independent of pressure, showing only a slight monotonic decrease with increasing pressure up to 15 atm. In the latter range, the process is essentially diffusion controlled and the pressure dependence can be explained by the concomitant action of two chemical equilibria, viz. ZnS+2HCl ⇄ ZnCl2+H2S and the dissociation of hydrogen sulfide H2S ⇄ H2+ 1 2S2. Experiments carried out with mixtures of HC1 and Ar, and of HC1 and H2S reveal effects on the transport rate. One arises from the change of diffusion coefficients in the case of HCL and Ar. In addition, there is the change of chemical equilibrium in the case of HCl and H2S. © 1962.