Abstract The diffusional isotope effect of a binary Lennard Jones liquid is calculated by molecular dynamics simulation. Mass differences of 20–40% were used. At equilibrium density, with decreasing temperature, a strong reduction of the isotope effect is found for both species. This indicates a marked increase of the collectivity of motion. On changing the density at a constant temperature the same effect is seen for the larger majority atoms, which shows that the density change is the main driving force behind the reduction of the isotope effect. Although the smaller minority atoms also show a drop in isotope effect with increasing density, the relation of the isotope effect to temperature and density differs between the two species.