3,7-Bis(N,N-diethylamino)-10-(phenylthio)carbonylphenoxazine (1a) and its analogs (1b-f) were synthesized and the color change after gamma irradiation was investigated. The color change of acetonitrile solutions of la ([1a](0) = 1.0 x 10(-3) M) after gamma irradiation was identified by naked eyes at the dose of 20 Gy. Phenylthio-substituted compound la showed more significant absorbance increase than phenoxy-substituted compound, 3,7-bis(NN-diethylamino)-10-phenoxycarbonyl-phenoxazine (2a). The C-S cleavage in la by gamma irradiation was revealed to occur more easily than the C-O cleavage in 2a. The sensitivity of the color formers (1a-f) to gamma rays was correlated with the stability of the thiyl radicals generated in the reaction.