In the course of the development of an oxidizer-based extruded composite propellant for use as gas generant, a simple model to describe and evaluate the actual porosity using simple methods was developed. It is briefly described and used to assess the influence of porosity on the burn rate of propellants produced. For the formulation studied the presence of microporosities does not seem to significantly influence the burn rate. On the other hand, volume fraction of porosities in extruded composition grains can exert a large influence and can increase considerably under thermal treatment with a resulting augmentation in burn rate.