We have carried out a comparison of experimental results and 59 G4 calculations of the enthalpies of formation Delta H-f(298K)0 and hydrogenation Delta H-hyd(298K)0 of alkenes from 4 to 12 carbon atoms. We present a systematic calculation of the cis-trans isomerization enthalpy and a systematic comparison of experimental enthalpies of hydrogenation determined in this laboratory with the computed results of the same hydrogenations. The linear function of experimental results for alkanes and the linear function of the computed values lead to a linear function for converting one to the other. This result is an alternative to the commonly-used "atomization method'' for finding the enthalpy of formation of an alkane relative to the thermodynamic reference state of the elements in their standard states. We calculate results in vector and matrix representation for compactness and consistency in treating large sets of computed G4 results. We show that the methods given here are sufficiently accurate so that reliable extrapolations to larger molecules can be made and that they are applicable to acids of biological interest, to alcohols, amines, and, in principle, to many other compounds such as polyenes, alkynes, and substituted alkanes. (C) 2014 Elsevier Ltd. All rights reserved.