Internal mixtures of montmorillonite, a clay component of mineral dust, with sodium chloride or ammonium sulfate were studied optically using cavity ring down spectroscopy. The effects of the addition of the clay to the optically observed deliquescence relative humidity (DRH) and water uptake of these salts were considered by investigating a series of different salt mass fractions. In most cases, montmorillonite alters the hygroscopic properties, lowering the DRH in comparison to the pure salt, and causes the particles to transition from solid to liquid at a lower relative humidity than is expected based on the salt alone. Predictions based on volume-weighted mixing rules were not accurate for most measurements around the DRH. We attribute deviations from theory to changes in the Gibbs free energy of the system caused by disturbances in the ion-ion interactions and lattice structure allowing water uptake prior to the DRH of the salt. Our optical results contradict some current measurements in the literature that suggest little change in the hygroscopic behavior of salts when insoluble mineral dust components are added.