The enthalpy of formation of indigo, as a solid, was reported in 1893. The enthalpy of sublimation at a mean temperature of 577 K (ca. 298 degrees C) was reported some 90 years later, and corrected herein to 298 K. These values were summed to result in a standard gas phase enthalpy of formation. We have also performed quantum chemical calculations at the B3LYP/6-311++ G(d, p) level and analyzed them using (not quite) isodesmic and isomerisation reactions. From these disparate calorimetric and computational approaches, we hereby present a recommended enthalpy of formation of gaseous indigo of (35 +/- 16) kJ.mol (1). (C) 2013 Elsevier Ltd. All rights reserved.