Experimental determination of the isothermal high-pressure equation of state (EOS) of composites is not feasible by using conventional diffraction techniques. To overcome this issue in the case of polymer bonded explosives (PBXs), composites made of an energetic material and a polymeric binder, we have expanded the applicability of the optical microscopy and interferometry technique previously developed in our group. To accommodate representative samples of a PBX with large grains, we modified the diamond culets of a diamond anvil cell to include etched micrometer-scale pits. This enabled us to measure the isothermal EOS of a PBX, namely, LX-17, up to 8 GPa. The results are compared with the EOSs of the constituent materials and previously published shock measurements. The technique employed in this study is not limited to PBXs and could be potentially used for the EOS determination of other materials ranging from composites to alloys and granular materials.