Novel crystalline forms of and a process for converting perylene-3,4:9,10-tetracarboximide of the general formula I to a form used as a fluorescent dye. The process including: a) dissolving or suspending the perylene-3,4:9,10-tetracarboximide in a solvent to obtain a solution or suspension; and b1) cooling the solution obtained in step a) to a temperature at or below the crystallization temperature of the perylene-3,4:9,10-tetracarboximide and, if the solvent is organic, removing excess solvent until the first crystals form, otherwise, adding water or dilute aqueous solutions of the solvent until the first crystals form, and maintaining the solution at that temperature for further crystallization; or b2) cooling the suspension obtained in step a) to or below the crystallization temperature when the temperature in step a) was above the crystallization temperature, and maintaining the suspension at the temperature for further crystallization to form one or more solvate crystals.