The line width (ΔH) of the proton resonance has been measured as a function of temperature and pressure for four homologues of the alkyl cyanobiphenyl series (5, 6, 7 and 8 CB) and also for methoxy-cyanobiphenyl, p-azoxyanisole (PAA) and quinquephenyl. Values of the nematic order parameter (S2 = < P 2(cos θ)>) may be obtained from the results by dividing them by a scaling factor (ΔH/S2), which can in some cases be deduced theoretically and in other cases obtained by reference to magnetic susceptibility measurements at atmospheric pressure. Slight changes which we have observed in the shape of the resonance line as opposed to its width suggest that the mean conformation of the molecule is liable to vary with temperature and pressure in cases where it has a flexible end chain, though not otherwise. In principle the scaling factor (ΔH/S 2) may also vary with temperature and pressure in such cases, but the variation appears to be rather slight in practice. Our results for 5 CB and PAA are in general agreement with those reported by Horn and Faber and McColl respectively, who also used elevated pressures. The magnitude of S 2 at the nematic-isotropic transition (S2c) decreases on compression for 5 CB, 7 CB and 8 CB, but for 6 CB and the other substances we have investigated it appears to remain constant