The difference between liquidcrystal and gas-phase values for the nuclear quadrupole coupling constant in Dz and HD is used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals are calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the averqe tield gradient. The results obtained arc in good agcement with c?lperimental values for hydrogen -and several other solutes. An understanding of the mechanism of orientational order is of fundamental importance in the theory of liquidcrystalline systems. At present the interactions responsible for orientational ordering are not well understood,and the study of small molecules dissolved in anisotropic environments provides a logical starting point for a systematic investigation. In recent studies of methane [l-3] and hydrogen [3,4] * the observed ordering is attributed to interactions which can be expressed in secondorder tensorial form, but the physical origin of these forces has not been established. Mechanisms previously suggested include interactions with the liquid crystal through dispersion forces [5] , the polarizability anisotropy [I], size and shape [6], and moments of inertia [7,8] of the solute molecule. However, these models do not adequately explain many experimental results. For example, it is difficult to understand the observed negative order parameters found experimentally for the solutes hy-* The vibrational corrections reported for the B/D ratio in ref. [4] are in error. The ratio at 298 K is -2459 and at 320 K is -24.58 for Dz_