A series of aroylacetaldehyde aroylhydrazones were prepared and characterized. Their uv and1H nmr spectra suggest the enol-imine structure rather than the keto-imine form. ThepKavalues of these aroylhydrazones were measured and correlated with the Hammett substitution constants. It was observed that benzoylacetaldehyde substituted in thep-position could be cyclized to form the 5-hydroxy-2-pyrazolines by refluxing in acidified ethanol, while formyldeoxybenzoin only gives the corresponding pyrazole due to steric requirements of the two phenyl groups.