Lithium bromide solution is used as a desiccant in air dehumidification systems. Liquid desiccant is a solution that facilitates the removal of humidity directly from the air. In this work, effectiveness of a LiBr based air dehumidifier was studied by correlating the vapor–liquid equilibrium data with a proposed thermodynamic model. For this, the nonelectrolyte Wilson nonrandom factor (N-Wilson-NRF) model and the Pitzer–Debye–Huckel formula were used to represent the contribution of the short and the long range ion–ion interactions. In particular, the proposed model assumed that the electrolyte solution is treated as a mixture of undissociated ion pairs and solvent molecules. The proposed equation of this study is valid for the temperature range of 20–35 °C and concentration range of 0.40–0.60 kg/kg. This relation was employed to estimate the equivalent humidity ratio, and then, the humidity ratio from the previous step was used to calculate the effectiveness of a LiBr based dehumidifier. The response surface methodology (RSM) was applied for the design and analysis of the dehumidification experiments. A quadratic model was implemented to predict the dehumidification effectiveness. This model studies the implications of four primary variables on the effectiveness of a dehumidification process. The optimal values to achieve the maximum effectiveness were found to be 32.5 °C for the air temperature, 0.0210 kg/kg for the air humidity ratio, 2.17 for the mass flow rate ratio, and finally, 0.50 kg/kg for the desiccant concentration. These values gave the dehumidification effectiveness of 0.544. The result of the model was in good agreement with the experimental value 0.542, thus verifying the accuracy of the proposed model.