Present paper concerns the effect of substituent groups of 2-acylaminophenol couplers on dark stability of three kinds of quinonemonoimine dyes by means of polarographic analysis. The dyes were prepared by oxidative coupling of CD-3 developing agent with the 2-acylaminophenols. Polarographic measurements of the dyes were carried out in a mixture of ethanol and Britton-Robinson buffer solution (1.5: 1 in volume ratio) as the supporting electrolyte at 20°±0.2°C, bubbled with nitrogen gas. Some experimental data showed: 1) Two protons took part in the fading reaction of these dyes. 2) In the case of Dye I, plots of E versus log { i /( i _d- i )} revealed that the fading reaction of Dye I was a two-electron process in the pH range lower than 7.0. The redox system of Dye I was expressed in the following equation: Dye I+2 e +2H⁺→Leuco Dye I. In the reduced dye solution with thiosulfate or EDTA-Fe (II), polarographic waves of Leuco Dye I and tetrathionate ions were observed separately. Accordingly, the chemical equation of the leuco dye formation of Dye I in the fading reaction with the reducing agents was expressed as follows: Dye I-I-2 Red.-1-2^H+→Leuco Dye I+2 Ox. 3) On the other hand, half-wave potentials of Dye II and Dye III were close to those of reducing agents. Therefore, polorographic waves of the leuco dyes and the oxides of the reducing agents were not observed separately to each other. 4) Half-wave potential values of these dyes were as follows: Dye I>Dye II>Dye III. These values correlated with the stability of the dyes. For example, Dye III is the most stable among the dyes under reductive conditions. The effect of substituent groups of these dyes on the half-wave potentials was discussed. Some basic information for predictions about dark stability of a quinonemonoimine dye image under dark condition was obtained by measuring the half-wave potential of the dye.