The equimolecular salt of piperazine and 1,2-diphenyl-4-butyl-3,5-dioxopyrazolidine is prepared by stirring together separately prepared solutions of the two compounds in the same organic solvent, selected among the ketones, ethers, toluene and chlorinated aliphatic hydrocarbons containing 1 to 3 carbon atoms and by seeding the reaction mixture by means of crystals of the equimolecular salt when temperature of the reaction mixture has decreased to a value at which the seeding crystals are not dissolved in said mixture and before any precipitate appears in said reaction mixture.
