A study on the composition of the resin component of the several brands of composite resin has not been published so far. It was the purpose of the present work to fill out this gap in our knowledge of this important group of restorative materials by analyzing the monomers of the materials as delivered by the manufacturers. 24 brands of restorative resins were investigated by means of a Nuclear Magnetic Resonance spectrometer at 90 MHz. Spectra were taken of deuterated chloroform solutions of the organic part of the filling materials and compared with spectra of pure monomers that were conceivable constituents of the investigated brands. It was found that the BIS-GMA containing brands all contain at least one other monomer, most often TEDMA, in varying concentrations. The brands with MMA as main component in most cases also contain other monomers.