Pyrazolone can exist in 3 isomers: 3-pyrazolone, 4-pyrazolone, and 5-pyrazolone. These isomers can interconvert via lactam–lactim and imine–enamine tautomerism; these conversion often display photochromism. For pyrazolone derivatives, the 5-pyrazolone form can be stabilized with N-alkyl or N-aryl substituents.