Rates of isomerization of 1H-indene-1-carboxylic acid to 1H-indene-3-carboxyiic acid were measured in dilute aqueous solutions of HClO4, NaOH, and CH3CO2H and H2PO4-, (CH3)(3)CPO3H-, and HCO3- buffers. This gave a rate profile which, together with the occurrence of general base catalysis and sizable primary kinetic isotope effects, indicates that the isomerization takes place through an enolization-reketonization reaction sequence. The equilibrium constant of the isomerization reaction is K = [indene-3-carboxylic acid]/[indene-1-carboxylic acid] = 200 in aqueous acid solution and K = 100 in base. The ratio of products formed by ketonization of the indenecarboxylic acid enol intermediate generated in the photolysis of 2-diazo-1(2H)-naphthalenone is R = [indene-3-carboxylic acid]/ [indene-1-carboxylic acid] = 0.47 in aqueous acid solution and R = 20 in base. The failure of previous investigations of the photolysis reaction to detect any indene-1-carboxylic acid as the product is attributed to the facile isomerization of this substance to indene-3-carboxylic acid and the preponderance of the latter at equilibrium. The enol intermediate of this isomerization reaction was also generated by flash photolysis of 2-diaza-1(2H)-naphthalene and rates of its ketonization were measured in dilute aqueous HClO4 solutions. Analysis of the data gave the enol acidity constant pK(a)(E) = 2.09. The results, in combination with those for the isomerization reaction, also provided carbon acid acidity constants (K-a(K)) and keto-enol equilibrium constants (K-E) for the two acids : pK(a)(K) = 9.35 and pK(E) = 7.26 for indene-1-carboxylic acid and pK(a)(K) = 11.69 and pK(E) = 9.60 for indene-3-carboxylic acid.