The density of Hanford tank waste supernatants affects the design and performance of waste treatment processes. The density of aluminate ion [Al(OH){sub 4}{sup -}] bearing sodium hydroxide solutions is important for describing the caustic leaching processes in the Hanford Waste Treatment Plant Flowsheet. The dissolved aluminate has a particularly large impact on the density of supernatants, but this ion is absent from most density estimation algorithms because of its rarity in most industrial processes. Fortunately, there is a large amount of published data on the density of aqueous sodium-hydroxide-sodium aluminate solutions, which can be used to develop density models. This study reviewed the available data and determined the partial molar volume of sodium hydroxide and sodium aluminate for mixtures of these salts in water by regression. This study determined that much of the published data suffered from a strong correlation between the sodium hydroxide and sodium aluminate concentrations in solution. Nonetheless, there was sufficient un-correlated data to identify and quantify the affect of both hydroxide and aluminate concentration on solution density. The density was found to increase linearly with both hydroxide and aluminate concentrations over a wide composition range. The effect of temperature on the density of aqueous sodium hydroxide-sodium aluminate solutions was found to be statistically significant but small in magnitude. (authors)