Salicylsalicylic acid and acetylsalicylsalicylic acid were identified as decomposition products of aspirin when mixtures of the drug, with magnesium stearate, were stored in the solid state of 60 degrees and 75% relative humidity. The effect of increasing the concentration of magnesium stearate and the addition of other alkali stearates on the rate of decomposition of aspirin were studied. The validity of the theory that pH changes induced by the alkali stearates account for the catalytic effect of the lubricants on the decomposition was tested. The changes observed were modeled and the mechanism involved elucidated. The potential use of the melting points of aspirin mixtures in predicting the stability of the drug in such drug-excipient mixtures is demonstrated.