The influence of an isotropic substrate on the anchoring properties of a nematic liquid crystal has been investigated by varying the thickness of a polyimide orientation layer. By invoking the screening effect of the polymer film on the anisotropic van der Waals interactions between the nematic and the solid substrate, the dependence of the pretilt angle on the thickness of the film has been theoretically evaluated. The agreement of the proposed theory with the experimental data is fairly good. ©1995 American Institute of Physics.