Digestion of AgBr emulsions with increasing amounts of reduction sensitizers achieved stepwise increase in sensitivity corresponding to the formation of two kinds of sensitization centers, designated R centers, acting as positive-hole traps, and P centers, acting as electron traps. It has been proposed that R and P centers are silver dimers formed at electrically neutral and positively charged surface sites on AgBr grains, respectively. The formation of P centers was depressed by addition of TAI and PMT, which are preferably adsorbed to positively charged surface sites on the grains, and enhanced by cyanine dyes, which are preferably adsorbed to negatively charged surface sites. Reduction sensitization of AgCl emulsions was less effective than that of AgBr emulsions in that the former brought about less increase in sensitivity and more increase in fog density than the latter with increasing amounts of sensitizers. Reduction sensitization of AgCl emulsions was significantly improved by digesting the emulsions for reduction sensitization in the presence of PMT and TAI. From the fact that the sensitivity increase of AgCl emulsions was not associated with a decrease in photoconductivity with photoelectrons as carriers and was not observed in the presence of hydrogen hypersensitization centers acting as positive-hole traps, it is considered that, in AgCl emulsions, reduction sensitization centers were solely R centers, while P centers acted as fog centers. The developability of P centers in AgCl emulsions was however very weak. It was found that R centers exhibited their absorption band, whose peak wavelength (420 nm) was close to that of the absorption band of Ag2 in rare gas (412 nm). This result supports the proposed model where an R center is a silver dimer at a neutral site that physically interacts with the surface of silver halide.