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Ultrafast spatial interferometric measurements of shock dynamics in transparent thin films exhibit phase shifts caused by both surface motion and the interference of multiple reflections off the moving shock wave interface. The interference effects are strong perturbations on the phase shift, which do not allow independent measurement of surface motion. Calculations of the time dependent phase shift that include reflective surface motion, shock wave transit through the transparent thin film, and thin film interference effects are shown to match experimental measurements in 625-nm-thick films of polymethylmethacrylate (PMMA) shocked to 19 GPa. Interferometric data obtained at two angles of incidence and two polarizations were sufficient to uniquely determine the PMMA shocked refractive index, shock speed, and particle velocity. Interferometric results as a function of shock strength, 2–20 GPa, suggest that submicron PMMA films have essentially the same material response to shock loading (Hugoniot) as macroscopic samples. © 2003 American Institute of Physics.