The variations of chemical exergy with ambient temperature from -30 degrees C to 45 degrees C, pressure from 0.6 to 1.1 bar and relative humidity (RH) from 10% to 100% were investigated. for gaseous fuels and atmospheric gases on the basis of Szargut's model. It was found that the variations are significant. At ambient pressure of 1 atm and RH at 70%, the chemical exergy of hydrogen increased 0.7-0.8% per 10 degrees C of lower ambient temperature, while for methane, the increase was 0.25-0.30%. For other gaseous hydrocarbons, the increase was 0.08-0.20%. An error analysis verified that the uncertainties in the results were 2-3 orders of magnitude less than the results. It was shown that assuming the atmospheric mole fraction of water vapor constant when varying the temperature leads to unrealistic and even unphysical results, giving an opposite behavior of the model. Calculating the change of chemical exergy over certain processes showed that separation of air gases is potentially most efficient in cold climates, while water electrolysis to hydrogen is favorable in warmer climates. Combustion reactors and fuel cells are potentially most efficient in cold climates. (c) 2007 Elsevier Ltd. All rights reserved.