[en] The treatment of mass spectra presented here was based on the hypothesis that following ionization and excitation, the excitation energy of the large molecule-ions was distributed in a random fashion by means of radiationless transitions among the numerous electronic states. On the basis of this hypothesis, the mass spectrum of a large molecule was considered to be the result of a series of quasi-equilibrium unimolecular decompositions of vibrationally excited molecule-ions. Appearance potential differences in various mass spectra were interpreted as heats of activation for unimolecular decomposition reactions. The reaction sequences leading to the formation of the principal ions in the mass spectra of butane and isobutane were deduced, and heats of reaction and heats of activation calculated for the various steps. The resulting schemes were found to be consistent with the scheme previously deduced for propane by Wallenstein and the heats of reaction and activation also were in essential agreement with those calculated by Wallenstein. A rate equation appropriate for the decomposition of isolated systems having a large but finite number of degrees of freedom was discussed and applied to the unimolecular reactions leading to the propane mass spectrum. Activated complexes for the various reactions were postulated and reasonable values of the physical parameters were assigned and used in quantitative calculations of the several rate constants. Lacking more exact knowledge, some very simple distribution functions for internal excitation of the parent molecule-ions were applied to an actual calculation of the propane mass spectrum, assumed to be the result of a series of unimolecular decompositions occurring in a time t = 10^-5 seconds. The excellent agreement between the calculated and observed mass spectrum was strongly indicative of the validity of the theory, although the numerical values of the parameters involved were very tentative. The effect of temperature on mass spectra was discussed in terms of changes in the energy distribution function for the parent molecule-ions. The effect of deuteration on rate of hydrogen loss in hydrocarbon mass spectra was discussed in terms of differences in non-localized energy among various isotopically substituted molecule-ions and differences in the frequency factors of the rate equations. Metastable transitions were shown to be a natural consequence of this theory since it postulated that mass spectra were formed by a whole family of such metastable transitions having a continuous range of half-lives; the small relative abundance of experimentally observed metastable transitions was shown to be due to experimental factors. A number of examples were discussed supporting the corollary that these metastable transitions would occur only for the lowest energy processes among a number of competing unimolecular reactions