The sensitivity of ultra-fine-grain (UFG) emulsions (minimum size 35 nm) was studied and was found to be proportional to the fourth and fifth powers of the grain size. The sensitivity decreased more than expected from the decrease in the absorption of light. A desensitizing factor peculiar to UFG emulsions was suggested. A large increase in the sensitivity was observed for low-intensity light exposure when a coated film was immersed in a sodium nitrite solution before exposure. Under these conditions, sensitivity became proportional to the third power of the grain size. Rehalogenation is the desensitizing factor in the UFG emulsion and the sodium nitrite solution functions as a halogen acceptor. Rehalogenation proceeds markedly in UFG emulsions because silver atoms and halogen atoms/molecules are formed nearby and could therefore react easily with each other. The sensitivity increase of the UFG emulsion due to sulfur-gold sensitization was smaller than that due to immersion in the sodium nitrite solution; both increases were additive for low-intensity exposure. In contrast, the sensitivity increases were inversely related (and not additive) for high-intensity exposure.