A robust computer model has been developed for the humidification of a high-pressure, hot, dry-air stream via contact with water in a countercurrent column. This algorithm was developed for use in the simulation of Humid Air Turbine power cycles. The solution technique is based on equation of state equilibrium calculations, mass and energy balances, and integrated mass and heat transfer expressions. Profiles of air temperature, air humidity and water temperature as a function of column height are provided for gas cooling or gas heating conditions. The algorithm also determines if the mass and heat transfer are rate-limited (unsaturated air) or equilibrium-limited (saturated air). The algorithm requires the specification of the input air and water streams to the column. A measure of the column’s capacity to provide mass and heat transfer must also be provided in the form of either (a) an Approach To Wet Bulb (ATWB), which is used to determine the temperature of the water exiting the column, or (b) the column characteristic. If the user provides an unrealistic ATWB or does not specify an ATWB or column characteristic, the algorithm determines the lowest ATWB that can be realized in the column. The solution algorithm provides the effluent air humidity, effluent air temperature, and (a) the column characteristic required to attain the specified ATWB, or (b) the ATWB which corresponds to the specified column characteristic.