The relative stabilities of 2- and 3-furancarboxylic acids have been studied, using an experimental thermochemical investigation and high-level ab initio calculations. The enthalpies of combustion and sublimation of 2- and 3-furancarboxylic acids were measured and the gas-phase enthalpies of formation at 298.15 K were determined. Standard ab initio molecular orbital calculations at the G2(MP2) and G2 levels were performed, and a theoretical study on the molecular and electronic structures of the studied compounds has been conducted. Calculated enthalpies of formation, using atomization and isodesmic bond separation reactions, are compared with the experimental data. Experimental and theoretical results show that 3-furancarboxylic acid is thermodynamically more stable than the 2-isomer. A comparison of the substituent effect of the carboxylic group in benzene, thiophene, and furan rings has been made.