The identification of the di- and tetrahydrogenated derivatives of methyl- and dimethyldicyclopentadiene isomers has been performed directly in the hydrogenated mixtures, by comparing the results from the gas chromatography–mass spectrometric mass balance with those from quantitative ¹³C nuclear magnetic resonance spectra. The assignment of the carbons of these isomers has been realized by analysing data from J-modulated spin-echo sequences and by applying semi-empirical rules for the influence of the methyl groups on the carbon chemical shifts in cyclopentane and norbornane ring systems.