The effects of poly(vinyl pyrrolidone) K30 (PVP) (1) on the solubility of lamotrigine (2) and diazepam (2) at 298.2 K are reported for ethanol (3) + water (4) mixtures at five different concentrations (0.0001, 0.0005, 0.001, 0.005 and 0.01) g·mL−1 of PVP. The general version of the Jouyban−Acree model was fitted to the solubility data of each drug in a given concentration of PVP. The largest mean relative deviation (MRD) was 14 %, that is, for diazepam in the presence of (0.0005 and 0.001) g·mL−1 of PVP, and the lowest MRD was 4 % for lamotrigine in the presence of 0.01 g·mL−1 of PVP. The overall MRD for lamotrigine and diazepam was 9 %. A modified version of the Jouyban−Acree model was used to model the simultaneous effects of the ethanol and PVP on the solubility employing experimental solubility of drugs in water and ethanol in the absence of PVP. The MRDs for diazepam and lamotrigine were 23 % and 10 %, respectively, with the overall value of 17 %.