In measurement of the solubility of organic compounds in mixed solvents containing water and miscible cosolvent(s), significant deviations from the widely used log-linear model have been observed. The deviations have been considered to be due mostly to the nonideality of the solvent mixture. In this paper, an approach to extend the log-linear model was evaluated. Activity coefficients of the solvent components were estimated using the UNIFAC group contribution method, and the sum of their logarithms weighted by either mole fractions or volume fractions was added to the log-linear model. Four forms of the extended-model were evaluated using a large database with 13 different cosolvents and over 4000 experimental solubility data. The best form of the extended log-linear model generated improvement of 0.06-0.22 log units for 7 of the 13 water-cosolvent systems over the log-linear model in estimating the solute solubility in mixed solvents.