Studies have been made on the mechanism of the sensitizing effect (supersensitization) of tetraazaindenols for spectrally sensitized and sulfur-sensitized AgBr emulsions. The sensitization by a tetraazaindenol was observable only when the tetraazaindenol was introduced into emulsions before their exposure to light by immersion treatment. The degree of the increase in the relative quantum yield of spectral sensitization was much smaller than that of the increase in sensitivity caused by the tetraazaindenol. These results seem to indicate that tetraazaindenols depress the recombination process introduced by sensitizing dyes in the sulfur-sesitized emulsions.