The mass spectrum of CD3—CH2—CH=CH2 has been measured in a 90° Nier-type mass spectrometer using 50-volt electrons. High resolution nuclear magnetic resonance measurements showed that the methyl group was fully deuterated and that there were no D-atoms located elsewhere in the molecule. This result was supported by infrared analysis. A comparison of the mass spectrum with that of CH3—CH2—CH=CH2 shows that the total intensities of each group of C4, C3, C2, and C1 fragments are the same for both compounds. This indicates equal probabilities of C—C bond rupture in the dissociation of the corresponding parent ions. The distribution of fragments within the groups in the deuterated compound shows, however, that extensive migration of the D-atoms has occurred during ionization. Migration is also evident at much lower electron energies (approximately 15 ev.). The thermal cracking patterns of the two compounds, as determined by gas chromatographic analyses, are very similar.