Analytical expressions for the corrections to the simple effectiveness-NTU method are developed using perturbation technique for cooling towers and liquid desiccant-air contact systems. The developed model takes into consideration the effect of nonlinearities of humidity ratio and enthalpy of air in equilibrium with water or desiccant solutions. The model also takes into consideration the effect of water loss by evaporation and the effect of variation of the specific heat capacity of water or solution. The comparison with numerical integration of the dimensional heat and mass transfer equations shows that the analytical results are generally satisfactory and the improvement over the simple epsilon-NTU method is significant. (C) 2007 Elsevier Ltd. All rights reserved.