We investigated the dynamics of the radical ion pairs formed by photoinduced electron-transfer reaction from zinc tetraphenyl porphyrin to 2-methyl-1,4-naphthoquinone in mixtures of 2-propanol and cyclohexanol. By the irradiation of a resonant X- (9.16 GHz) or Ku-band (17.41 GHz) microwave pulse, the time profiles of the transient absorptions was modified and the yields of escaping radical ions decreased. From these experiments, we determined the kinetic parameters of the radical ion pairs at various solvent viscosities. The recombination rates of the singlet pairs were (4 +/- 4), (8 +/- 3), and (16 +/- 3) x 10(6) s(-1) at 5, 10, and 15 cP, respectively. The escape rates were (1.7 +/- 0.2), (1.4 +/- 0.1), and (0.9 +/- 0.2) x 10(6) s(-1) at 5, 10, and 15 cP, respectively. The viscosity dependence of the kinetic parameters was followed by the simple continuum diffusion model.