A predictive equation, based on the rough-hard-sphere theory, was developed for : the determination of infinite-dilution molecular diffusion coefficients in supercritical fluids. The equation parameters were determined from molecular dynamics simulations and from experimental data on 1-octene diffusion in supercritical ethane, propane, and hexane over a wide range of temperatures and pressures for supercritical fluid densities in the range of 69.9-500.2 kg/m(3). An extensive database on diffusion in supercritical fluids was collected through the literature in order to evaluate the proposed equation and to determine the accuracy of the predictions with no additional parameter adjustment. The predictions were very good, with an average absolute deviation of 15.08% over 1548 data points. For systems where actual data on the critical constants were available, the average error of the predictions reduces to 9.61% over 939 points.