A thermodynamic model based on changes in Gibbs free energy and availability is developed for onset of heterogeneous condensation on subcooled rigid and liquid surfaces in moist air. Effects of wettability of the wall, subcooled wall temperature, relative humidity and dew point temperature of the surrounding air on critical radii for heterogeneous condensation on a rigid surface are investigated based on the change of Gibbs free energy. From the analysis of changes in availability, it is found that onset of droplet condensation is easier to occur on a hydrophilic surface than on a hydrophobic surface, in moist air than in a dry air environment, on a colder surface than a warmer surface, on a liquid surface than a rigid surface. It is also found that condensation droplet occurs earlier on oil surfaces with small surface tensions than on oil surfaces with large surface tensions. (C) 2014 Elsevier Ltd. All rights reserved.