Polyacrylonitrile (PAN) microfibrils were prepared directly via the solution polymerization of acrylonitrile at specified ratios of controlling solvent dimethyl sulfoxide (DMSO) to non-solvent tert-butyl alcohol. At first, gel formation occurred due to the interaction between DMSO and the cyano groups of PAN. The microfibrillar structure was then formed through the phase separation of PAN molecules from the gel. It is shown that very small variations in the solvent/non-solvent ratio facilitate a major change in the gelation and phase separation processes.