Quantitative infrared analysis in the carbonyl region with a simple curve-fitting procedure and thermomechanical measurements were used to analyse critical behaviours in naturally aged oil/resin mixtures covering a wide range of composition, The results are discussed on the basis of a comparative study performed with fresh mixtures of the same composition, used as standards, and a phase miscibility analysis. Thermomechanical measurements give macroscopic evidence for phase segregation in naturally aged mixtures with a resin component content strictly below 50%, while above this threshold the basic components are totally involved in the formation of a stable mixed oil/resin phase. At the molecular scale, infrared measurements appear to be sensitive to the subsequent oil/resin interaction in aged mixtures which appears to develop at the first stage of the mixing.