An important element in the microstructure of high performance fibers and films fabricated from rigid polymers is an interconnected network of oriented microfibrils, the lateral size of which is about 10 nm. This study is an attempt to elucidate the mechanism by which the microfibrils are formed so that larger lateral dimensions can be achieved by suitable processing. Because this morphology emerges in the coagulation stage of the spinning process, we compared the microfibrillar network formed by drastically different coagulation conditions. Ribbons, spun from solution of poly(p-phenylene benzobisthiazole) in polyphosphoric acid through a slit die, were coagulated either in the ordinary rapid process with water (timescale of seconds) or in a slow process with phosphoric acid (timescale of hours). The coagulated microfibrillar network was dried with supercritical CO2 for X-ray scattering measurements and impregnated with epoxy resin for sectioning and imaging by TEM. Slow coagulation yields better-aligned microfibrils of enhanced chain packing, but the lateral size of the microfibrils formed in both cases is similar, about 10 nm. Heat treatment increases the width of water-coagulated microfibrils but not the acid-coagulated ones. The observations do not support spinodal decomposition as the mechanism of microfibril formation during coagulation, as was previously suggested.