The supercritical fluid extraction of a low polymer using supercritical fluids with a linear density program is modelled. It shows that this density program gives an approximately constant polydispersity in the polymer being extracted at a particular time. Experiments are carried out on the supercritical extraction of polyisobutene and polydimethylsiloxane which show qualitative agreement with the predictions, both with respect to the shapes of the curves and also the effect of density ramp speed.