Equilibration of 1,9- and 7,8-C(70)H(2) has allowed the relative free energy of these isomers to be measured. These "simplest hydrocarbon derivatives of C(70)" are formed by hydroboration of C(70) at room temperature. Analysis of the platinum-catalyzed equilibration of these isomers yielded a relative free energy at 295 kelvin of 1.4 +/- 0.2 kilocalories per mole, with the 1,9 isomer being more stable. This value is in excellent agreement with the ab initio HF/6-31 G(*) calculated energy difference of 1.3 kilocalories per mole, whereas semiempirical calculations gave poor agreement.