We study the effects of composite microstructure on the dynamic response of Ultra-High Molecular Weight Polyethylene (UHMWPE) fiber-reinforced composites when loaded in out-of-plane compression. Using a Kolsky bar, we dynamically load the composites (of differing microstructures) in out-of-plane compression while simultaneously capturing the deformation history with high speed video with μs temporal resolution and approximately 10μm spatial resolution. Image analysis is performed on the resulting high speed video to obtain the finite deformation kinematics of the specimen, including the dynamic lateral deformations, and obtain the complete deformation gradient history during the tests. Using a finite deformation formulation, we measure the evolution of the volumetric deformations and relate this to porosity evolution within the specimen. Specimens are also tested at quasistatic strain rates and their response is compared in the range of strain-rates from 10−3s−1 to 3000s−1. These materials are quite compressible, and their volume can change very rapidly during out-of-plane compression. The volume change is accommodated by a reduction in porosity at low to moderate strains. In the uniaxial stress samples, high strains lead to volume change by ejection of material, and the evolution of material ejection is rate dependent. In both microstructures, deformation at the higher strain rate led to material ejection being initiated at higher stress, ejection evolving more slowly as a function of applied stress, and developing a plateau at a lower stress (than in the quasistatic case). At quasistatic strain-rates, there was little difference between the response of the two microstructures; however at high strain rates of 103s−1, the microstructure with lower porosity displayed a higher strength. The rate sensitivity of the strength was shown to depend on the microstructure, suggesting the defect distribution plays a significant role in the dynamic strength of these materials.