Experimental infinite dilution activity coefficients gamma (infinity)(1) for benzene and toluene in water and in aqueous solutions of two protein denaturants, urea and guanidine hydrochloride, are reported. The quantities were measured in the temperature range (273 to 323) K. Four denaturant concentrations were used from (0.5 to 5.0) mol.dm(-3), covering the range where proteins undergo chemical denaturation. For both solutes in water, from the freezing temperature of water, gamma (infinity)(1) rises with increasing temperature, passes through a maximum close to room temperature, and then continuously decreases with increasing temperature. Based on the results reported here and selected literature values, recommended correlations of gamma (infinity)(1)(T) for benzene and toluene in water were established. They clearly show that from the freezing temperature to the normal boiling temperature of water, the origin of the hydrophobicity of the non-polar solutes changes from being entropic to enthalpic in nature. The presence of a denaturant (modifier) produces a significant decrease of gamma (infinity)(1) values, their maximum being displaced towards lower temperatures. The effect is more pronounced for guanidine hydrochloride, which acts as a better solubilizing agent than urea. The solute in (water + modifier) gamma (infinity)(1) data were analysed by a classical thermodynamic scheme employing solution and transfer {water to (water + modifier)} quantities. The accuracy of the data allowed the derivation of enthalpies, entropies, and heat capacities and a detailed discussion of the several observed trends. A remarkable feature is that there is only a narrow temperature interval where both the enthalpic and entropic contributions to the Gibbs energy are favourable to solute transfer. At lower and higher temperatures, the enthalpic and entropic components compete, solute transfer being favoured by entropy or by enthalpy, respectively. Extrapolation of this behaviour suggests that the effect of the modifier on the solubility of hydrophobic solutes should eventually invert at both lower and higher temperatures. The data presented here might be used to understand better, through the application of different models, the exposure of non-polar amino acid side chains from the protein interior to the aqueous environment, which characterizes protein denaturation. (C) 2000 Academic Press.