X-band electron spin resonance was employed to study the structural anisotropy in several polythiophene derivatives. Because of the dominating homogeneous width, the obtained absorption spectra were Lorentzian-shaped. Information about the structural anisotropy was obtained from the position and width of the absorption peak. Qualitatively, the anisotropy was in full agreement with earlier results from X-ray diffraction, including a flip in molecular orientation with respect to the film substrate between solution-cast and spin-cast films. With the Monte Carlo technique, the spectra were fitted with a biaxial g tensor, an anisotropy parameter S, and an intrinsic width sigma. The simulations showed that g could be treated as pseudo-uniaxial, with the unique axis along the side chains rather than along the ring normal. Closed-form analytical expressions of the molecular anisotropy. Because the molecular g tensor for these materials was not known, a known value of S for one of the samples obtained by X-ray diffraction was used for normalization. Fairly consistent values were obtained for both g and S. (C) 2003 Wiley Periodicals, Inc.