The solute-solvent interactions of four anticancer drugs, daunorubicin, doxorubicin, vincristine sulfate, and 6-thioguanine, have been studied in acetonitrile-water mixtures up to 50 % acetonitrile by volume fraction using a UV/pH titration method. The acidity constants have been calculated with the STAR (stability constants by absorbance readings) program. The interactions between the four anticancer drugs and the solvent studied, acetonitrile-water mixtures, was identified using the microscopic parameters (Kamlet and Tafts solvatochromic parameters: alpha, beta, and pi*). The Kamlet and Taft general equation for pK(a1) and pK(a2) values of 6-thioguanine, doxorubicin, daunorubicin, and vincristine sulfate was reduced to two terms (the independent term and the hydrogen-bond-donating ability, a) or three terms (the independent term, polarity/polarizability, pi*, and the hydrogen-bond-donating ability, a) using linear regression analysis in acetonitrile-water mixtures. The Kamlet and Taft equations can be used to predict the acidity constants of daunorubicin, doxorubicin, vincristine sulfate, and 6-thioguanine at any acetonitrile composition, which would be helpful in practical work during chromatographic method development.