Abstract This work reports on raw biogas purification method via swollen polyamide thin-film composite membranes. Experiments on permeation of gas mixture through two commercial thin-film polyamide composite (TFC) membranes were performed using an in-house permeation apparatus. The active polyamide top layer of TFC membranes was swollen by water present in a feed stream of raw biogas, whose relative humidity was higher than 85%. An effective CO2/CH4 separation was based on the significantly higher solubility of carbon dioxide in water compared to that of methane. The transport properties of both composite membranes are discussed together with the structure of membrane top active layer. One-dimensional mathematical model for flow and mass transport in the membrane cell was developed. The model enables the evaluation of the mass transport coefficients by the iterative fitting of experimental data in the co-current and counter-current flow arrangements. The model also determines concentration profiles of gas component on both sides of the membrane, which are otherwise immeasurable experimentally. The model can be used to evaluate the effect of changing the membrane area on the performance of the membrane module. Model results are discussed with respect to the required CH4 enrichment.