The Hanford Waste Treatment Plant (WTP) flowsheet includes an optional caustic leach step to remove gibbsite (Al(OH){sub 3}) from high-level waste sludge prior to vitrification. Aluminum leaching minimizes the mass that must be vitrified as high-level waste. The steady-state (time averaged) WTP flowsheet uses thermodynamic models that minimize Gibbs free energy to predict aluminum dissolution in the caustic leaching process, but these models are too computationally intensive to be solved in dynamic flowsheets. A gibbsite solubility model that is both accurate and rapidly solved by was needed and developed for a dynamic flowsheet. Available literature data on the solubility of gibbsite in aqueous sodium hydroxide solutions was compiled and the apparent equilibrium constant (Q) was calculated from the experimental data for each data point. The Q value is defined as the true equilibrium constant times the activity coefficients for the aluminate (Al(OH){sub 4}{sup -}) and hydroxide (OH{sup -}) ions in the reaction: Al(OH){sub 4}{sup {r_reversible}} Al(OH){sub 3(s)} + OH{sup -} The WTP dynamic flowsheet uses Q to determine the concentration of dissolved aluminate at equilibrium with gibbsite for a given hydroxide concentration. An empirical model to predict Q was developed by multi-linear regression of the experimentally determined Q values from the literature. Four statistically significant model coefficients (all P statistics were less than 10{sup -25}) were identified: temperature, solution ionic strength, ionic strength squared, and a regression constant. This model was found to fit a large database of aluminum solubility data with an R{sup 2} of 0.98, which is comparable to the accuracy of more computationally intensive thermodynamic models for this data set. (authors)