The film wetted area is a significant parameter affecting the heat and mass transfer of falling film liquid desiccant air-conditioning system. In this paper, a theoretical model with the analytical solution was developed for calculating the wetted area accurately, by solving the desiccant interface concentration and temperature gradients. Comparing with the experimental data on a single channel internally heated regenerator, the results with the new model showed better agreement than those of the previous model. The average error reduced significantly from 16.4% to 4.8% for the experiments with different solution temperatures, and decreased from 10.8% to 7.0% for those with different plate surface temperatures. With this new model, decreasing the film thickness and contact angle could significantly increase the film area, and the area also increased with the air and extra fluid temperature. This research is useful to help to predict the wetted area accurately and to increase the area for improving system efficiency of liquid desiccant air-conditioning system and other falling film applications. (C) 2014 Elsevier Ltd. All rights reserved.