Translational diffusion constants of transient radicals produced by photochemical reactions of pyrazine, alpha-naphthoquinone, quinoxaline, and benzophenone with triethylamine (TEA) have been measured in trifluoromethane from rho(r) = 1.4 to 2.0, and alpha-naphthoquinone with TEA in carbon dioxide from rho(t) = 1.2 to 1.93, where rho(r) is the reduced density by the critical density of the solvent. The diffusion constants of the transient radicals almost linearly depend on the reciprocal of the solvent viscosity in the measured density region. The diffusion constants of the radicals are relatively smaller than those of stable molecules of similar molecular sizes, and the difference becomes smaller with increasing the molecular size, as is observed in liquid solution. However, the difference is smaller than that observed in the liquid solvent, which suggests that the effect of the solute-solvent attractive interaction on the diffusion constant is smaller in fluids at the medium-density region than in liquids.