Solutions of polyacrylonitrile (PAN) in dimethylformamide (DMF) sprayed into supercritical fluid carbon dioxide form hollow fibres and highly oriented microfibrils (<1 mu m diameter). In the dilute region, microfibrils are produced with diameters as low as 100 nm due to the dipole-dipole forces, in contrast with microspheres produced from solutions of polystyrene (PS) in toluene. For PAN microfibrils, orientation increases with shear, then goes through a maximum and eventually decreases at higher flow rates due to an expanding jet. The concentration for the transition from microfibrils to a single hollow fibre is in agreement with the calculated transition concentration from the dilute to semidilute region, C*. In the semidilute region, the morphology changes from hollow fibres to highly oriented fibrils with an increase in flow rate. The increase in turbulence enhances convective mass transport, leading to more uniform nucleation throughout the cross-section of the jet, favouring the highly oriented fibrils. The enhanced transport of CO2 into the jet lowers the solvent quality, raising C*, which further favours fibril formation. For both PAN-DMF and PS-toluene solutions, the transition from highly oriented microfibrils to hollow fibres occurs at about 3C* (in a good solvent), suggesting some similarities in the mass-transfer pathways in each system.