A single-crystal X-ray diffraction study showed that 1-phenyl-3-methyl-4-formyl-4,5-dihydropyrazol-5-one occurs in the solid phase as two conformers of the NH tautomer. The acidity and basicity constants of 1-phenyl-3-methyl-4-formyl-4,5-dihydropyrazol-5-one and its analog, -5-thione, in 50% aqueous dioxane were measured. PM3 calculations qualitatively explan the differences in the tautomerism and acid-base properties of 4-formylpyrazolone (-pyrazolethione) and the corresponding compounds without the formyl substituent.