The structure of the product obtained by reacting phenylhydrazine, p-chlorophenylhydrazine and hydrazine itself with dehydroacetic acid has been established by a combined use of X-ray crystallography and NMR (1H and 13C) spectroscopy. The X-ray structure of 1-phenyl-3-methyl-4(1-phenyl-3-methyl-1H-pyrazol-5-yl)-2H-3-pyrazolin-5-one1d has been determined. The compound crystallizes in the space group P21 a [C20H18N4O, a = 11.801(2), b = 7.911(1), c = 18.363(2) A, α = 90.00°, β = 100.04°, γ = 90.00°, Z = 4]. The structure corresponds to the NH-tautomer of the pyrazolinone moiety; the angle of twist corresponding to pyrazole rings, defined as C1-C2-C4-C5, is -119.1(7)°. NMR spectrum allows to estimate the tautomeric equilibrium constant in solution and in the solid state. The X-ray structure of compound 1 (NH-tautomer) was determined and compared with NMR results (solution and solid state). © 1995.