Large voids of about 100 mu m are often found in rigid-rod polymer thin films coagulated from isotropic solutions. To understand the mechanism of this void formation so as to improve the optical quality of coagulated rigid-rod polymer thin films, surface and internal structures of various coagulated poly(p-phenylene benzobisthiazole) solutions were investigated on freeze-dried films by high resolution scanning electron microscopy. The films coagulated immediately from doctor-bladed solutions exhibited surface crinkles and large internal voids, while the films coagulated from the doctor-bladed solutions that had undergone a surface treatment in an atmosphere containing the vapour of a non-solvent showed a surface network structure but no large internal voids. Results support that the large voids were formed due to a large-scale coagulant incursion during coagulation. The absence of the large voids in the films coagulated from the surface-treated solutions is attributed to the surface network structure being strong enough to regulate the incursion of coagulant during the film forming process at coagulation.