A simple statistical model of interacting non-rigid molecules, based on a perturbation expansion of the pair correlation function and the additivity of segmental interactions, is applied to the study of orientational order as measured by N.M.R. in the nematic and SAphases of 4-n-alkyl-4′-cyanobiphenyls (N-CB, N = 5 to 8), the nematic and SCphases of 4-n-alkyloxybenzoic acids (N-OBA, N = 7, 8) and theDh0columnar discotic phase of hexa-alkyloxytriphenylenes (N-THE, N = 5 to 8). The order parameters of each homologous series are correctly described in terms of two isotropic and two anisotropic segmental coupling constants. The model predicts certain relations among coupling constants pertaining to different homogous series. These predictions are supported by the results obtained for the three types of compounds studied.