By use of a basic catalyst, lithium β-aminoethylamide in ethylenediamine–benzene, trans-Δ¹- and trans-Δ²-octalins have been equilibrated at three temperatures, and free energies, enthalpies, and entropies of isomerisation have been deduced. Similar but less precise information has been obtained for the interconversion of the corresponding cis-Δ¹- and -Δ²-octalins. The thermodynamic data are compared with those resulting from other experimental work on related steroid olefins and with the theoretical predictions of quantitative conformational analysis. For the process trans-Δ¹- octalin →trans-Δ²-octalin, as dilute solutions in ethylenediamine–benzene (2 : 1), our best values are ΔH=–0·52 ± 0·06 kcal./mole, ΔS=–0·8 ± 0·3 cal./(deg. mole); for the cis-isomers corresponding estimates are +0·25 ± 0·25 kcal./mole and –0·8 ± 0·7 cal./(deg. mole), respectively.