Gas chromatography (GC) with a Flame Ionisation Detector (FID) has been used to determine changes in the concentrations of the components of Otto Fuel II (OF) in contact with an 82% aqueous solution of hydroxylammonium perchlorate (HAP) in sealed vials at 31.7 degrees C during the period leading up to auto- ignition of the two liquids. The concentration of hydroxylamine in HAP was monitored over the same period using a titration method. It was found that 2-nitrodiphenylamine (2NDPA), the stabiliser in the OF, is completely consumed after about 65-70 h and that the concentration of hydroxylamine begins to fall at this point. 1,2-Propanediol dinitrate (propylene glycol dinitrate, PGDN), the energetic component in the OF, is not depleted significantly until after about 90 h. The evolution of nitrous oxide (N2O) between 65 and 90 It is attributed to the reaction of the hydroxylammonium ion with nitrous acids produced by PGDN decomposition at the liquid-liquid interface. Carbon dioxide (CO2) is evolved after similar to 90 h and is attributed to PGDN decomposition. HAP and PGDN are each thought to contribute to N2O evolution after similar to 90 h.