The initial step of a reaction sequence initiated by a strong acoustic shock has been observed for the first time in a condensed phase energetic material. Shocks are generated by pulsed laser vaporization, of thin aluminum films and launched into adjacent films of glycidyl azide polymer at 77 K through an inert spacer material. Changes in the composition of the polymer films are monitored by transmission FTIR spectroscopy, Comparison of spectra obtained before and after the shock reveals that the initial reaction sequence involves elimination of molecular nitrogen from the azide functional groups on the polymer.