The invention relates to a process for the manufacture of 1-hydrazono-isoindolines of the formula ##STR1## wherein Y is a methine or imine radical and the ring A can contain substituents which do not confer solubility in water, by reacting an iminoisoindoline of the formula ##STR2## with hydrazine or a hydrazine donor, in a polar solvent, which process comprises carrying out the reaction at temperatures below 80.degree. C.These hydrazono-isoindolines are obtained in excellent yield and purity and can be used without further purification as starting materials for further syntheses.