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This paper describes research into the low strain rate mechanical properties of polymer bonded explosives (PBXs) and follows on from that presented by Palmer and co-workers in 1993. PBXs are highly filled composite materials comprised of crystals of a secondary explosive supported in ca. 5-10% (by mass) polymeric binder. In general, the modulus of the binder is 10 5 times lower than that of the crystalline explosive, resulting in the behaviour of the composite being heavily influenced by the properties of the binder despite the low concentration present. The Brazilian test, in which a disc of material is loaded diametrically in compression, has been used to generate tensile failure in the materials studied. Three methods of microscopy have been used to examine the nature of failure in two UK compositions and one US material (PBX 9501). Pre-and post-failure optical and electron microscopy examinations of the materials have been undertaken to gain a greater understanding of the role of the microstructure and this has been aided by the use of an environmental scanning electron microscope to follow real-time failure in the PBXs. Failure in all three compositions has been observed to start around the edges of larger filler particles perpendicular to the direction of tensile strain. Compositions with rubbery binders have been observed forming binder filaments which bridge the crack walls, while clean crystal faces are observed on the larger particles. In a composition with a nitrocellulose-based binder, hair-like features of nitrocellulose have been seen sticking out from the failure surfaces and the rougher crack walls.