The free energy difference between the two chair conformations of trans-1,2-dichloro- and -1,2-dibromo-cyclohexane (II) and trans-1,4-dichloro- and -1,4-dibromo-cyclohexane (III) has been determined by peak area n.m.r. measurements in a number of solvents at low temperature. Both equilibria show a strong solvent dependence [despite the absence of a dipole moment for either of the interconverting species in (III)] and this is given a quantitative explanation in terms of a known theory of solvation. From this the vapour phase free energy differences [ΔG(aa→ee)] have been estimated as 0·9 and 1·5 kcal mol^–1(II; X = Cl and Br) and 0·8 and 0·9 kcal mol^–1(III; X = Cl and Br), i.e. in all cases the diaxial conformer is preferred in the vapour phase. In the trans-1,2-dihalogenocyclohexanes this is interpreted on the basis of steric (repulsive) X ⋯ X interactions in the diequatorial conformer. The extra stability of the diaxial conformer of (III), over that of the corresponding cyclohexyl halides is shown by calculations of both dipole–dipole and charge–charge interactions between the C–X bonds not to arise from this mechanism. Attractive 1,3-polar interactions between the positively charged axial hydrogen atoms and the axial halogens are shown to account for the stabilisation of the axial conformers in the 1,4-compounds.