AbstractWhen a photographic emulsion is sensitized in the presence of increasing concentrations of the restrainer 7-hydroxy-5-melhyl 5-triazolo-(l, 5-o) pyrimidine, silver sulphide formation is at first retarded, but later accelerated. With increasing concentrations of the restrainer I-phenyl-tetrazoline-5-thione, however, sulphide formation is at first accelerated and later retarded. These phenomena can be largely explained by a consideration of the differences in solubility products of the silver salts of the two restrainers. The effect of the two restrainers on light sensitivity is also considered.