N-Phenylmaleimide (PMI) was used as an acrylonitrile (AN) solution which is convenient to handle because PMI is an irritative solid and difficult to handle. However, the PMI solution tends to color during storage upon heating and its yellow color is apt to become brownish. Impurities contained in the colored PMI solution were isolated and their structures identified. Azobenzene (AZB) and N,N’-diphenylhydradine (DPH) are the compounds which cause the color change in the solution. The extent of the coloration of the PMI solution increases with increase in the contents of AZB and DPH. It is considered that AZB and DPH are produced by the oxidation of aniline (ANL) generated by the hydrolysis of PMI in the solution. The addition of the antioxidants under a low concentration of oxygen was examined to prevent coloration of the solution based on the proposed mechanism of the coloration. These conditions proved to be sufficiently effective for stabilization of the solution. Furthermore, it was also confirmed that the extent of coloration of the polymer prepared from the stabilized solution is effectively minimized.