The Onsager heat of transport Q* has been measured for CO2 at the surface of aqueous ammonia. The heat of transport incorporates the enthalpy of reaction of gaseous CO2 with ammonia, adsorbed on the liquid surface, to form adsorbed ammonium carbamate, with the result that -Q* has the unusually large value of 180 kJ mol (1). Measurement of Q* for transfer of a reactive species through a surfactant monolayer is proposed as a new method of studying reactions at liquid and quasi-liquid surfaces. (C) 2010 Published by Elsevier B.V.