Quantitative infrared analysis in the carbonyl region with a simple cu
rve-fitting procedure and thermomechanical measurements were used to a
nalyse critical behaviours in naturally aged oil/resin mixtures coveri
ng a wide range of composition, The results are discussed on the basis
of a comparative study performed with fresh mixtures of the same comp
osition, used as standards, and a phase miscibility analysis. Thermome
chanical measurements give macroscopic evidence for phase segregation
in naturally aged mixtures with a resin component content strictly bel
ow 50%, while above this threshold the basic components are totally in
volved in the formation of a stable mixed oil/resin phase. At the mole
cular scale, infrared measurements appear to be sensitive to the subse
quent oil/resin interaction in aged mixtures which appears to develop
at the first stage of the mixing.