The solubilities of benzothazolium-based ionic liquids (ILs) [HBth][BF4], [HBth][CH3SO3], and [HBth][p-TSA] in binary ethanol + ethyl benzoate solvents were measured using the equilibrium method with the temperature range from 233.2 to 293.7 K at atmospheric pressure. Results of these measurements were correlated by both lambda h equation and the modified Apelblat equation. It was found that both models gave satisfactory correlation results. In the lambda h model, the average relative deviations were 0.7145, 0.7845, and 0.8119% for [HBth][BF4], [HBth][CH3SO3], and [HBth][p-TSA], respectively. Interestingly, the parameters of lambda and h for the lambda h model and A, B, and C for the modified Apelblat model were simultaneously expressed as functions of solvent composition (x(sol)) for the first time. Furthermore, solubilities of these three ILs in the binary system were predicted through these two sets of self-created correlation equations with ideal performances. The total average relative deviations were less than 2.2% for both models. These successful in-depth correlations and validated prediction results have guaranteed the accuracy of experimental solubility data and provide us a reliable method for accurate solubility prediction at any temperature with any solvent composition. This developed calculation method in this report lays a foundation for accurate industrial recovery of ILs and gives us valuable guidance for the design of the specific purification process to acquire desired purity of the product in industrial production.