Flash photolysis of a series of arylhydroxycyclopropenones (aryl = phenyl, mesityl, 4-methoxyphenyl, 2,4,6-trimethoxyphenyl, and 1-naphthyl) in aqueous solution was found to give arylacetic acids as final products through two successively formed, short-lived reaction intermediates. Rate profiles, the form of acid-base catalysis, and solvent isotope effects identified the first of these intermediates as arylynolate ions, ArC=CO-, and the second as arylketenes, ArCH=C=O; the identity of the ketenes was also confirmed by independent generation through photo-Wolff reaction of corresponding aryl diazo compounds. The kinetic behavior of the arylynolate ions shows that the corresponding arylynols, ArC=COH, are remarkably strong acids, with pK(a) < 3.