A theory is presented whereby expectation values for the anisotropic molecular polarisability tensor, and thence of the molar Kerr constant (averaged over allowed configurations), of n-alkyl halide molecules can be calculated. Only staggered configurations of the n-alkyl halide chain are allowed, and tetrahedral bond angles are assumed. Configurations are weighted according to gauche/trans energy differences and excluded volume considerations. The treatment is applied to n-propyl, n-butyl, n-pentyl, and n-hexyl bromides and iodides, satisfactory agreement between theory and experiment being obtained. For n-propyl bromide and iodide, gauche/ trans energy differences (in kcal./mole) are -0.26±0.5 and +1.31±0.5, respectively.