Experiments have been conducted using a low pressure laboratory flame apparatus to examine the chemistry of solid rocket motor (SRM) afterburning relevant for stratospheric altitudes. It was found that a significant fraction of the HCl injected into H2-O2 and H2-CO-O2 flames can be consumed, with observed losses of up to 40%. The extent of conversion of HCl was found to increase with increasing oxygen:fuel (O/F) ratio and decreasing pressure; the loss at a given O/F was also higher for flames with equal flows of H2 and CO compared to flames with no CO in the fuel. The major product of HCl reaction was found to be Cl2, with no other chlorine-contained products observed via mass spectrometry. Distinct Cl2 B -> X emission bands were observed along with very weak ClO A -> X bands and a bright, white continuum emission that apparently arose from one or more chlorine-containing compounds. The general findings concerning the magnitude of HCl conversion and the formation of Cl2 are consistent with published modeling results for SRM stratospheric afterburning. This formation of "free" chlorine could lead to catalytic destruction of ozone in regions near the path the launch vehicle follows during boost through the stratosphere.