2-Monosubstituted-3-hydroxy-5-pyrazolones were prepared from diethyl malonate itself and diethyl malonates monosubstituted with methyl, ethyl, propyl, butyl, amyl, hexyl, heptyl, and benzyl groups by condensation of the esters witho-,m-, andp-chlorophenylhydrazines, andn-hexylhydrazine. By using diethyln-hexyl malonate-2-C14ando-,m-, andp-chlorophenylhydrazines, andn-hexylhydrazine as starting materials the corresponding pyrazolones labelled with C14were obtained. Their specific activities were 7.0, 8.8, 9.0, and 8.8 µc./gm. respectively. Ultraviolet absorption spectra were determined in neutral and alkaline solution and the infrared spectra were also obtained. From the data it was possible to ascribe the tautomeric structures best suited for the compounds.