The possibility of using Raman spectroscopy to determine the local thermoplastic content within blends of epoxy resins and copolyethersulfone was studied. The required calibration of the Raman data is presented here. It was obtained by multivariate methods and was based on homogeneous samples of known content in amine-ended thermoplastic (expressed in weight %). Blends cured with two types of diamine curing agents were separately studied, the curing agents being the methylene bis(2,6-diethylaniline) (MDEA) and the 4,4'diaminodiphenylsulfone (DDS). Partial least-squares (PLS) regressions with external validation were performed. With MDEA, root mean square error of prediction (RMSEP) values at validation were 3.2, 3.7, and 3.0% (depending on the spectral pre-treatment and on the wavenumber selection). RMSEP values were 2.4 or 3.0% with DDS (depending on the wavenumber selection). The applicability of the calibrations to blends containing chlorine-ended copolyethersulfone is discussed. Results obtained with small or large Raman confocal holes are also compared.