Methods have been developed to determine by Raman spectroscopy the epoxide conversion in a 4,4'-diaminodiphenylsulfone (DDS)cured epoxy resin as well as in blends of this resin with copolyethersulfone thermoplastic. The proposed spectral treatments consisted of a peak ratio method and of multivariate analysis. The Raman data were first successfully calibrated on the basis of neat resin samples with the use of the values of epoxide conversions provided by differential scanning calorimetry (DSC) measurements. The use of DSC as a reference technique is discussed, In order to be applied to blends of the same epoxy resin and reactive copolyethersulfone, the calibrations were based on spectral data that would not be influenced by the presence of this thermoplastic. The blend samples were homogeneous at the scale of the Raman laser focusing. The epoxide conversions deduced from the Raman and the DSC results were compared for these blends. Limitations associated with both techniques are discussed.