A series of 5, 5-dimethyl-4-aryl-Δ1-1, 2, 4-triazolin-3-ones (Ar= C6H5, p-C6H4CH3, p- C6H4OCH3, p-C6H4Cl, and p-C6H4Br) were prepared from the corresponding 4- arylsemicarbazones of acetone by oxidative cyclization on alumina. The triazolinones decompose in solution to N2, CO, and isopropylidene aryl amine, with first order kinetics, in the temperature range 148-200° C. Average activation parameters are Δ H≠= 35 kcal mol ...