Films of the organic semiconductor 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) are deposited at room temperature in ultrahigh vacuum on the (2×4)‐c‐(2×8) As‐terminated, and the (2×1) Se‐passivated (100) GaAs surfaces. The PTCDA deposition on the (2×4)‐c(2×8) surface produces domains randomly oriented in the plane parallel to the interface, giving rise to diffuse ringlike low energy electron diffraction (LEED) patterns. A marked improvement in the PTCDA molecular order is observed for the growth on the Se‐passivated substrate. The resulting LEED patterns are sharp, and indicate that the interface molecular unit cells are azimuthally oriented with respect to the Se‐dimers. The improvement in the crystallinity of the PTCDA layer is attributed to the termination of the chemically active sites by Se and to the smoothness of the Se‐passivated surface. This work provides information as to the conditions under which the quasiepitaxial growth of a lattice mismatched van der Waals film oriented to the substrate can be achieved. ©1995 American Institute of Physics.