Addition of a small amount of L-ascorbic acid (AA) unexpectedly provided uniform-sized polystyrene (PS) spheres in an ordinary dispersion polymerization process. PS spheres incorporated with polyimide prepolymer (BANI-M), designed as possible spacers for liquid crystal displays, were prepared from formulations containing a mixed solvent of isopropanol/2-methoxyethanol, 2,2 -azobisisobutyronitrile, and poly-N-vinylpyrrolidone (PVP) at 343 K. The average particle size decreased by the addition of AA; however, dramatic improvements of monodispersity were observed with various formulations. The best coefficient of variation was 1.37%, compared to 7.03% obtained without the presence of AA. Addition of AA was originally intended to scavenge a trace amount of oxygen remaining in the reactor system, promoting a rapid and short particle nucleation period, and thereby resulting in improved monodispersity. However, there was no difference in the initial reaction rate between the runs with and without AA; no noticeable induction period was observed even without AA provided that an ordinary procedure was employed to eliminate the dissolved oxygen through a nitrogen bubbling, and the nitrogen atmosphere was maintained during the reaction. It was proposed from 1 H-NMR analysis that the abstraction of hydrogen atom from the PVP chain by the oxidized form of AA promoted the formation of graft copolymer PVP-PS, which stabilizes precipitating chains in the nucleation period. Approximately 40% of BANI-M present initially was incorporated in the PS particles, which occupied 4-9% of the weight of polymer particles, well below the target value of 20%.