A continuous process for producing crystalline ammonium sulfate, said process comprising a start-up operation followed by a steady-state operation, wherein the start-up operation comprises: i) in a crystallizer, evaporating solvent from an approximately saturated ammonium sulfate solution; ii) replacing evaporated solvent with further approximately proximately saturated ammonium sulfate solution; iii) introducing to the crystallizer seed crystals of ammonium sulfate; iv) continuing to evaporate solvent, until a desired degree of supersaturation is reached; and v) recovering crystalline ammonium sulfate from ammonium sulfate solution in a recovery unit, and the steady-state operation comprises: vi) continuously feeding approximately saturated ammonium sulfate solution into the crystallizer and continuously withdrawing ammonium sulfate crystals from the crystallizer, such that the total combined volume of ammonium sulfate solution and ammonium sulfate crystals within the crystallizer remains constant; and vii) recovering crystalline ammonium sulfate from ammonium sulfate solution in a recovery unit, characterized in that the degree of supersaturation in the crystallizer during the start-up operation is maintained between 1.2% and the point at which primary nucleation occurs; and apparatus suitable for carrying out the process.