Diffusion processes of intermediate radicals created by the photoinduced hydrogen abstraction reactions of ketones, quinones, and N-hetero aromatic molecules in ethanol and 2-propanol are studied at various temperatures by using the transient grating (TG) method. The temperature dependences of the translational diffusion coefficients (D’s) of both the radicals and the parent molecules can be expressed by the Arrhenius relationship. The activation energies (E(D)) for diffusion of the radicals are larger than those of the parent molecules and the difference in ED depends on the molecular size. The different ED is explained in terms of the molecular volume dependence of ED; that is, larger molecular volumes of the radicals could be the cause of the larger E(D). The larger apparent molecular volumes of the radicals are consistent with a model of microscopic aggregation of the surrounding molecules around the radical.