Co-solvents and solid dispersions with polyvinyl pyrrolidone were tested to increase solubility of thiabendazole. Solid dispersions were prepared by the solvent method and analyzed by differential scanning calorimetry. The solubility was measured at 15-35 degrees C in aqueous (ethanol-water) and non-aqueous (ethanol-ethyl acetate) mixtures. Combination of solid dispersions with cosolvents increased the water solubility of thiabendazole in a larger extent that each method separately. The effect of the solid dispersions is greatest in water and it decreases nonlinearly as the volume fraction of ethanol-in water increases. The solubility enhancement is smaller in ethanol-ethyl acetate and is uncorrelated with co-solvent concentration. Solubility parameters delta were used to predict drug/carrier compatibility and related to solubility profiles. Thiabendazole shows an intermediate behaviour between solubility curves with two peaks (more polar drugs with larger delta values) and a single peak (less polar drugs with lower delta values). The solid dispersions increase the solubility parameter of thiabendazole from delta=24 to delta=25.7 MPa(1/2). The model of Bustamante et al. allowed solubility prediction including jointly both mixtures whereas the equation of Jouyban et al. was able to predict the solubility at several temperatures in each binary mixture separately, using a few experiments.