A semi-empirical approximation of the generalised Percus-Yevick (PY) e
quation for a mixture of hard spheres by Lebowitz (1964) is used to im
prove the representation of the thermodynamic properties of dense flui
d mixtures. The radial distribution function obtained by Lebowitz has
been used to derive mixing rules in which a difference of the size of
the constituent molecules has a direct effect. The performance of the
new approximation is illustrated by comparing the results obtained fro
m it with those from two earlier models for fluid mixtures with differ
ent molecular sizes within the framework of the principle of correspon
ding states. In order to separate the examination of the effect of cha
nges in the mixing rules from the effects of uncertainties in the pair
potential for real fluids, the tests conducted here make use of the r
esults of computer simulation for prescribed and simple intermolecular
potential models.