The formation of formic acid via the reduction (hydrogenation or electrochemical methods) of carbon dioxide (CO2) has been receiving great attention, not only in the context of the production of renewable fuels or synthons, but also since it is a convenient carrier of hydrogen gas in the form of a liquid at ambient conditions. In this report, in order to help elucidate the effect of solvent on the thermodynamic parameters for the hydrogenation of CO2 in water and acetonitrile, the acidity constant for formic acid in acetonitrile was determined experimentally, for the first time, to be 20.7 +/- 0.2 at 25 degrees C, and also the standard Gibbs energy changes for the transfers of formic acid and formate anion from water to acetonitrile were established.