We have investigated how the effectiveness of a corrosion inhibitor added to an aqueous solution to suppress the corrosion rate of steel is reduced by the addition of sand. The equilibrium adsorption isotherms of the inhibitor with respect to both the steel surface (consisting of iron carbonate under the corrosion conditions used here) and the sand surface have been measured. The results enable the quantitative calculation of how the surface concentration of inhibitor at the steel surface is reduced by sand addition. Combining the adsorption information with measurements of how the steel corrosion rate depends on the inhibitor surface concentration enables the quantitative prediction of the inhibitor effectiveness as a function of sand concentration. Excellent agreement is obtained between calculated and measured values of the inhibitor performance as functions of both inhibitor and sand concentrations. This methodology demonstrates how the optimization of a corrosion inhibitor formulation for specific application conditions should take into account the parasitic adsorption of the inhibitor onto the competitor surfaces present.