The chemical evolution of blends of poly(vinyl methyl ether) and polystyrene in conditions of photooxidation has been studied by infrared spectrometry and the photoproducts formed have been identified. The characterisation of the oxidation kinetics has permitted the detection of interactions between the two polymers. Analysis of the photooxidised samples by AFM has shown that the changes of the surface aspect could be characterised as a function of the irradiation time using several parameters and a tentative correlation between the modifications of the surface and the phase separation with the evolution of the chemical structure is proposed.