An acrylonitrile solution of maleimide manifesting transparency, precluding coloration and opacification during the course of handling, and excelling in stability, a method for the preparation of the acrylonitrile solution, and an acrylonitrile based copolymer obtained by using the acrylonitrile solution are provided.The acrylonitrile solution of maleimide is such that, when it is subjected to a forced coloration test, the differences .DELTA.L, .DELTA.a, and .DELTA.b values (invariably as absolute values) each between L, a, and b values before and after the test are respectively not more than 5, not more than 5, and not more than 10. The acrylonitrile solution of maleimide is prepared by lowering the water content in the acrylonitrile solution to a level of not more than 0.1% by weight, or causing the acrylonitrile solution to permit coexistence therein of at least one member selected from the group consisting of alkyl-substituted hydroxybenzenes and hindered phenols and at least one member selected from the group consisting of phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides, or lowering the water content in the acrylonitrile solution to a level of not more than 0.3% by weight and causing the acrylonitrile solution to permit coexistence therein of at least one member selected from the group consisting of alkyl-substituted hydroxybenzenes, hindered phenols, phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides.