Polymer matrix particulate composites comprise the majority of solid-phase high explosives, and most show considerable deterioration in mechanical properties when damaged. We have quantified this effect in three RDX-HTPB composites by measuring four damage metrics; the change in compressive strength, modulus, thermal conductivity and porosity, with increasing impact strain (the causal metric). This approach was applied to three RDX-HTPB composites containing (a) coarse, (b) fine and (c) both coarse and fine particles. Damage was applied by compression to predetermined levels of strain at strain-rates of the order of 10(3) s(-1). All three materials exhibited deterioration in strength and storage modulus, but only the composites containing coarse particles showed significant changes in porosity or thermal conductivity with increasing specific impact energy. This is because the fine-particle composites mask internal damage by recovering their initial form, the resultant cracks are closed, whereas the coarse particles underwent grain reorientation when impacted and are left open. This last point highlights the difference between active and passive assessments of damage since the former do not change the density or thermal conductivity of the material. The deterioration of secant modulus that results from damage can also be predicted using a simple energy-activation model.