Diffusion constants of organic molecules in the excited triplet states are measured in solution by using the transient grating technique with a high wavenumber, which is about 1 order of magnitude larger than that commonly used so far. Using this high wavenumber grating system under two different experimental conditions, we can detect the diffusion process of molecules in the T-1 states as long as the lifetime is longer than 300 ns. Contrary to the previously observed slow diffusion of many transient radicals, diffusion coefficients (D), of all molecules in the T-1 states of this study (benzophenone, 4,4'-dimethylbenzophenone, phenazine, anthracene, fluorenone, 2-nitrofluorenone, benzil, 1-nitronaphthalene, C-60, and Michler's ketone) are close to those in the ground state but most of them (except C-60) are slightly (less than or equal to 5-10%) but certainly larger than D in the ground states. D of C-60 in the T-1 state is found to be the same as that in the ground state.