The approximately in vitro constant release rate shown by 'multiple un
it' modified release systems that consist in coated minimatrices seems
to depend on swelling properties of the cores and on permeability and
/or mechanical characteristics of the applied polymeric film. The pres
ent study investigates the overall release mechanism by comparing the
release kinetics of systems based on inert or swellable cores and by m
easuring the permeability of isolated membranes whose composition is i
dentical to that of the applied coating. The results indicate that sai
d coating is not capable of working as a limiting diffusional step; co
nsequently, the release control can reasonably be ascribed to the phys
ical restriction exerted by the film on core swelling.