By extrapolation of experimental data on the entropies and heats of formation of the lower hydrocarbons, a set of rules is deduced for estimating the entropy and heat of formation of any paraffin or olefin hydrocarbon at 25°C. These properties are tabulated for all paraffins through the octanes and for all olefins through the hexenes.These values can be used to calculate the approximate state of equilibrium in any hydrocarbon reaction. The equilibrium proportions of 2,3-dimethyl-l-butene, 3,3dimethyl-l-butene, and 2,3-dimethyl-2-butene at 300°C. are calculated to be 25.4:2.2:72.4, compared to Whitmore's experimental ratio of 31:3:61. Equilibrium calculations for the paraffins show that in the temperature range 150°to 175°C. all the isomers in any group of isomers have approximately the same free energy. The more highly branched isomers are more stable below this temperature range, and the straight-chain and less highly branched ones are more stable above this temperature range.