AbstractThiokol HC 434 carboxyl‐terminated polybutadiene on hot storage in air increases in viscosity and molecular weight until, finally, gelation occurs. It was found that rate of aging can vary greatly from lot to lot. The difference in aging between lots is thought to be caused by a decrease in the catalytic activity of metal ion impurities due to chelation by varying amounts of glutaric acid remaining in HC 434 after manufacture. Stability of poorer‐aging HC 434 is improved by adding glutaric acid or other chelating age