The reaction of 2,5-disubstituted-2,4-dihydro-3H-pyrazol-3-ones (1) with acetone gives rise to the formation of 2,5-disubstituted-4-(1-methylethylidene)-2,4-dihydro-3H-pyrazol-3-ones (2), which further react with phenylisocyanate by the catalysis of triethylamine to afford 1,3-disubstituted-4-methyl-6-oxo-1,6-dihydropyrano[2,3-c]pyrazoles (3) in high yields. The reaction mechanism of the conversion and the role of the tautomeric character of 2 in this conversion are described.