PEDOT:PSS is a water-dispersable  and electrically conductive polymer blend that is increasingly applied in numerous fields such as batteries and super-capacitors. While many studies focus on performance optimization, degradation issues because of humid environments are rarely discussed. PEDOT:PSS absorbs significant amounts of water (~50 wt%), which leads to a pronounced film swelling factor of up to 1.6. The integration of PEDOT:PSS into a cellulose nanofibril (CNF) matrix enhances significantly the mechanical integrity and prevents film swelling, whereas a certain water amount is still absorbed into the PEDOT:PSS/CNF films (~24 wt%). By studying the water dynamics via QENS under varying ambient relative humidity (RH) conditions, we identified two water species inside the films: fast-moving bulk water and slow-moving hydration water. Under dry conditions, bulk water is completely released from the films, while some of the hydration water remains within them. In humid environments, both water species are present. The altered water content inside the PEDOT:PSS/CNF films in dependence of RH, leads to changing water-cellulose interactions, structural re-arrangements, and also affects the electrical conductivity: Under dry conditions, only some hydration water is present in the films and PEDOT:PSS – CNF interactions become more dominant. As a consequence, PEDOT:PSS wets on the CNF, thereby leading to an increased conductivity. In high RH conditions this is reversed: the high water content inside the films promotes de-wetting of PEDOT:PSS and the electrical conductivity decreases. In addition, the QENS measurements provide detailed information about the diffusive behavior of water molecules, while the PEDOT:PSS/CNF film morphology was analyzed with small angle neutron scattering under grazing-incidence (GISANS).