  The phase behavior of a system composed of spherical particles with a
monomodal size distribution is investigated theoretically within the context of
the van der Waals approximation for polydisperse fluids. It is shown how the
binodals, spinodals, cloud-point and shadow curves as well as all the
(polydispersity induced) critical points can be obtained for a variety of
interaction potentials. The polydispersity induced modifications of the phase
diagram (even for a polydispersity index $I$ as small as $I\approx 1.01$)
should be observable in some colloidal dispersions.
