Pyrazolone derivatives are prepared by condensing one or two molecular proportions of a pyrazolone with xanthydrol, alloxan, or triketohydrindene hydrate. 1 - Phenyl - 3 - (a - phenylbutyrylamino) - 4 - xanthydrylyl-5-pyrazolone is prepared by refluxing together in acetic acid 1-phenyl-3-(a -phenylbutyrylamino) - 5 - pyrazolone and xanthydrol. 1-(21 : 41 : 61-Trichlorophenyl)-3-[311-(2111 : 4111-di - tert. - amylphenoxyacetamido) - benzamido] - 4 - xanthydrylyl - 5 - pyrazolone, 4 : 41 - triketohydrindene - bis - 1 - (21 : 41 : 61 - trichlorophenyl)-3 - [311 - (2111 : 4111 - di - tert. - amylphenoxyacetamido) - benzamido] - 5 - pyrazolone, and alloxan-bis - 1 - (p - tert. - butylphenoxy) - phenyl - 3 - m - (p - tert. - amylphenoxybenzoylamino) - 5 - pyrazolone are similarly prepared. 4 : 41 - Alloxan - bis - 1 - (p - tert. - butylphenoxy) - phenyl - 3 - a - (p - tert. - butylphenoxypropionylamino) - 5 - pyrazolone is prepared by refluxing together 1 - (p - tert. - butylphenoxy) - phenyl - 3 - a - (p - tert. - butylphenoxypropionylamino-5-pyrazolone and alloxan monohydrate in pyridine. 4 - (4 - Hydroxyalloxan) - 1 - phenyl - 3 - methyl - 5 - pyrazolone is prepared by adding alloxan monohydrate in small portions to a suspension of 1-phenyl-3-methyl-5-pyrazolone in distilled water. 4-(4-Hydroxy)-triketohydrindene - 1 - phenyl - 3 - methyl - 5 - pyrazolone is similarly prepared. Specifications 524,154, 524,554, 524,555, 541,589, [all in Group XX], and 680,488 are referred to.