The relative jump frequencies of tracer atoms with masses 1.00 and 1.05 times the host atom mass have been computed by molecular dynamics in a system of 255 Lennard-Jones atoms comparable with solid argon at 80 °K. The chief error of previous theoretical treatments of the isotope effect is found to be their neglect of unsuccessful saddle point crossings (e.g. U turns) which become more prevalent the lighter the diffusing atom. In the system studied, this effect lowers the ΔK factor from its harmonic value of 0.98 to an effective value of 0.89±0.05. © 1975.