In this laboratory experiment, students synthesize 3-methyl-4-(4-methylbenzoyl)-1-phenyl-pyrazol-5-one by selective C-acylation of 3-methyl-1-phenyl-1H-pyrazol-5-one. Calcium hydroxide is used to push the tautomeric equilibrium toward the enol form, to protect the hydroxyl functionality as a complex, to trap the liberated hydrogen chloride, and to keep the reaction media basic. The product is obtained in excellent yield and recrystallized from various solvents and solvent systems. The chromatographic and physical parameters are determined and the results are analyzed. The signals in the NMR spectra of acyl pyrazolone are assigned and then compared with those of three previously synthesized acyl pyrazolones and two corresponding O-acylated products.