A mixture of acridine and a pyrazolonederivative was reacted in the solid state (without solvent). It is proposed that the enol tautomer (the C4-position) of the pyrazolonederivative attacks the C9-position of acridine through a nucleophilic reaction resulting in products where the C4-position of pyrazolone is connected to the C9-position of acridine. When the reaction of 3-methyl-1-phenyl-5-pyrazolone