Ultrafast laser shock generation methods and broad-band infrared reflection absorption spectroscopy have provided evidence that shock-induced chemistry in a condensed-phase energetic material occurs on a time scale of tens of picoseconds and involves the nitro group as a primary reactant. Femtosecond broad-band infrared reflection absorption spectroscopy was used to monitor films of the energetic polymer poly(vinyl nitrate) during shock loading and rarefaction. At similar to 18 GPa, poly(vinyl nitrate) films exhibited loss of absorption in the nitro group stretch modes that did not recover upon rarefaction, providing an indication of initial chemical reaction. At pressures less than or equal to9 GPa, the observed spectral changes are ascribed to thin film optical interference effects, without chemical reaction. The loss of infrared absorption required an induction time of tens of picoseconds after shock passage, supporting reaction mechanisms that require vibrational energy transfer rather than prompt reaction.