The heats of co mbustion of ten C4 hydrocarbons were meas ured b y combustion of t he gaseous hydrocarbon in a flame at constant pressure in a glass reaction vessel in a calorimeter. The calorimeter system was calibrated by means of electrical e ne rgy.The exp erimental data yielded the following values for the standard heat of combustion, -t:J.Hco , at 25° C and co nstant press ure, of the gaseous hydrocarbon in gaseous oxygen to form gaseo us carbon dioxide and liquid wate r, with all t he reactants and products in their thermodynamic standard reference states, in k cal/mole : n-butane, 687 .42 ± O.15 ; 2-methylpropane, 68S.37 ± O.11 ; I-butene, 649.33 ± O.18 ; cis-2-bute ne, 647.6S ± O.29 ; trans-2-butene, 646.90 ± O.23 ; 2-methylprope ne, 645.19 ± O.2S ; 1,2-butadiene, 619.93 ± O.13 ; 1,3-butadiene, 607.16 ± O.18 ; I-butyne, 620.64 ± O.20 ; 2-buty ne, 61S.84 ± O.23 .Values of hea ts of isomerization, formation, and hydrogenation were derived from these dat a .