The athermal model of a binary rectangular-parallelepiped fluid, continuous in translations and discrete in orientational distribution, is used for calculations of infinite dilution activity coefficients, gamma(infinity), of nonmesogenic solutes dissolved in nematic and isotropic phases of model nematogens. The particles having D-2h symmetry were chosen so as to model molecules of meta- and para-xylenes (nonmesogens) and the first five homologues of 4,4＇-alkyloxyazoxybenzenes (nematogens). The calculations of y(i)(infinity) (i = m,p denotes isomers of xylene) for various values of the relative temperature, t *, have explicated different characters of the functions y(i)(infinity)(t *) for the isotropic and nematic phases. The ratio gamma(m)(infinity)/gamma(p)(infinity) is less than 1 and practically constant in the isotropic phase, whereas in the nematic phase it holds that gamma(m)(infinity)/gamma(p)(infinity) > 1, the ratio increasing with a decrease in t *. These features agree with experimentally established regularities and confirm the significance of the effect of the particle biaxiality in interpreting the separation of structural organic isomers with the aid of nematic sorbents. (C) 1998 Elsevier Science B.V. All rights reserved.