The surface chemistry of colloidal silica was investigated in relation to its application as a stabilizer for a hydrolabile drug, aspirin, in tablets. Water vapor adsorption properties of the selected silicas were studied as a function of vapor pressure at 40°. Silica I, a precipitated colloidal form, exhibited a type IV adsorption isotherm, characteristic of multilayer adsorption with limited pore volume. Therefore, the Brunauer, Emmett, and Teller (BET) theory of multilayer adsorption was employed to quantify the monolayer capacity (Xm) and the specific surface area available for water vapor adsorption (Sw). The surface area and average particle diameter of I, determined by BET nitrogen adsorption, were 714.4m2/g and 3.8nm, respectively. Silica I, which had the greatest moisture adsorption capacity, was then evaluated for its stabilizing effects on aspirin in tablet form. Since silica increased the tablet void space in proportion to its concentration, Control (aspirin) and sample (aspirin–silica) tablets with controlled void space (held constant at 20%) were subjected to accelerated stability evaluation. In general, silica enhanced aspirin stability; a concentration of 3% offered maximum stabilization. Tablets with higher silica concentrations (up to 15%) showed poorer stability and approached the control tablets in aspirin content at the end of 120 days.