Apparent molar heat capacities ϕ_C have been measured in water + organic-solvent mixtures at both ends of the concentration range at 10, 25 and 40 °C. The ϕ_C of water in the following organic compounds shows a maximum against concentration with dϕ_C/dT < 0: acetone, tetrahydrofuran, methylacetate, ethylacetate and methylethylketone. For water in diethylamine, ϕ_C decreases monotonically with increasing water concentration and dϕ_C/dT < 0. In all systems at this concentration end the excess heat capacity C^E_p is large and positive with dC^E_p/dT > 0. The data are explained using an extended version of the Treszczanowicz–Kehiaian (TK) model for associated mixtures in terms of water self-association and complex formation through hydrogen bonds with the proton-acceptor organic solvent. The water-diethylamine behaviour reflects especially strong complexing. Water is suggested to have similar thermodynamic behaviour to a lower alcohol when at low concentration in an organic solvent. This is supported by positive C^E_p data for methanol in methylacetate. Interpretation with the TK model predicts that C^E_p should be negative for a higher alcohol, e.g. decan-1-ol, in methylacetate and this is confirmed. At the water–rich end, ϕ_C for the organic substances decreases linearly with the concentration of the organic solvent, and C^E_p is again large and positive but dC^E_p/dT < 0. The data are inconsistent with traditional concepts of aqueous solutions of hydrophobic solutes, where water structuring around the hydrophobe lowers solution stability. They support the views of Shinoda that water structuring has the opposite effect. The Shinoda treatment provides an interpretation for the LCST in aqueous solution and is consistent with the thermodynamic effect of structure in alkanes.