A molecular dynamics simulation was made of a system of 5 solute molecules in 495 solvent molecules interacting via the Lennard-Jones 12-6 potential, in order to study solvent density effects on the diffusion coefficients in supercritical fluids. The effects of the size of the solute and the strength of the solute-solvent attractive interaction on the diffusion coefficient of the solute were examined. The diffusion coefficients of the solute molecules were calculated at T = 1.5 (in the Lennard-Jones reduced unit), slightly above the critical temperature, from ρ = 0.1 to ρ = 0.95, where ρ was the number density in the Lennard-Jones reduced unit. The memory function in the generalized Langevin equation was calculated, in order to know the molecular origin of the friction on a solute. The memory function was separated into fast and slow components. The former arose from the solute-solvent repulsive interaction, and was interpreted as collisional Enskog-like friction. The interaction strength dependence of the collisional friction was larger in the low- and medium-density regions, which was consistent with the 'clustering' picture, i.e., the local density enhancement due to the solute-solvent attractive interaction. However, the slow component of the memory function suppresses the effect of the local density on the diffusion coefficients, and as a result the effect of the attractive interaction was smaller on the diffusion coefficients than on the local density. Nonetheless, the solvent density dependence of the effect of the attraction on the diffusion coefficient varies with the local density, and it was concluded that the local density was the principal factor that determines the interaction strength dependence of the diffusion coefficient in the low- and medium-density regions (ρ < 0.6).Molecular Dynamics Simulation of Solute Diffusion in Lennard-Jones Fluids. Yamaguchi, T., Kimura, Y., Hirota, N.: Molecular Physics, 1998, 94[3], 527-37