Poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is a widely used conductive polymer (CP) for applications in electronic devices. In the context of transparent electrodes or soft actuators, the uptake of water by PEDOT:PSS is an essential element in the performance of the physical system. We study the water uptake of pure films and films treated with ethylene glycol (EG) commonly used to enhance the electrical properties of PEDOT:PSS. Gravimetric analysis was used to investigate the water sorption-desorption of PEDOT:PSS and its change with the EG treatment for a wide range of configurations (thickness, temperature, and relative humidity). We demonstrate that a simple Fickian model cannot correctly represent the experimental results; we therefore introduce a fully coupled reaction-diffusion scheme. This model describes the transport of diffusing molecules into the polymer film, taking into account, in addition to the classical diffusion mechanism, the reaction between the reactive sites of the polymer network and the water molecules. We demonstrate that solvent treatments have a direct influence on the kinetics of the water uptake of PEDOT:PSS films in terms of diffusivity, solubility, and rate of reaction. The proposed model can be used to accurately predict the water uptake of CP films in the case of complex three-dimensional configurations that are needed for the design of complex actuators.