The invention concerns a novel process for the racemization of unwanted enantiomeric forms of 1'-substituted spiro[imidazolidine-4,3'-indoline]-2,2',5-triones and 1'-substituted spiro[pyrrolidine-3,3'-indoline]-2,2',5-triones which comprises heating an unwanted enantiomeric form (or a mixture containing an excess of one enantiomer over the other) alone or, more conveniently, in the presence of a solvent or diluent at a temperature in the range 80.degree. to 280.degree. C.