The effect of excipients on the stability of salsalate in suspension is studied in an attempt to model the decomposition of drugs in the solid state. The zero-order rate of formation of salicylic acid is shown to be increased by the addition of alkali stearates, an effect which is produced by alkaline impurities rather than by the pure stearates. Thus, a direct relationship between the concentration of magnesium ions and the rate constants for salicylic acid formation, was found to be secondary to pH changes induced by impurities in the stearates. By following the decomposition of salsalate in both suspensions and solutions, it is shown that the first-order solution degradation rate constants, derived from zero-order suspension rate constants and solubility data, were close approximations to those observed. The potential and limitations of suspension systems for predicting solid state stability are discussed.