Measurements by Trappenier and coworkers (Trappeniers, N. J., 1982, Ad
v. molec. relax. interac. Processes, 24, 297; Arends, B., Prins, K. O.
, and Trappeniers, N. J., 1981, Physica Al 107, 307) on the self-diffu
sion coefficient of fluid methane and ethylene at a number of temperat
ures over extensive ranges of densities are compared with theory. This
is based on the possibility of a consistent hard-sphere representatio
n of the experimental data, introduced by Trappenier et al., and on a
modern kinetic theory of dense hard sphere fluids.