The ionization and dissociation of propadiene, 1,3-butadiene, 1,3-trans-pentadiene, 2,4-hexadiene and 2,5-dimethy1-2,4-hexadiene have been studied using a CEC-21-701 mass spectrometer. The ionization potentials measured show good agreement to that reported previously by other workers. For 2,4-hexadiene and 2,5-dimethylhexadiene, whose appearance potentials have not been measured before, the results are in close agreement with the calculated values of their ionization potentials using the group orbital method. The appearance potentials of the principal ions from these compounds were measured and heats of formation of the ions were calculated when a reaction mechanism could be assigned. For processes where more than one reaction path was possible, the mechanism assigned was on the basis of the evidence of metastable ion transitions that could be correlated to the reaction in question. By a comparison of the heat of formation of ions produced from different sources, it is sometimes possible to decide whether excess energy is required for its formation from the alkadiene under examination.