A review is given of the Prigogine-Flory theory and its prediction of the first and second order excess thermodynamic quantities of non-electrolyte mixtures. Effects outside the scope of this theory occur when the mixing process : (1) destroys structure existing in the pure components (orientational order between n-alkane chains, dipolar and quadrupolar order) or (2) creates order in the solution (concentration fluctuations, alcohol multimers, hydrophobic hydration, micelles). These thermodynamic effects are most apparent in the second order quantities. A qualitative interpretation can be given.