We report on the modification of ferroelectric hysteresis in Pb(Nb,Zr,Ti)O-3 thin films under the adsorption of CO2 gas. The samples were exposed to the gas in ultra high vacuum while different voltages between the top (Ag) and bottom (Pt) electrodes were applied. After dosing, the samples were heated from room temperature at 1.8 K/s, and a mass spectrometer was used to detect the desorbed molecules. The effective molecular sticking was demonstrated by the appearance of the carbon Auger peak in the surface of the sample and by the presence of CO2 in the desorption curves. After +(-) 9.9 V were applied to the bottom electrode during dosing, we found a shift of 0.52 (+0.58) mu C/cm(2) in the remnant polarization. These evidences suggest the existence of a depolarizing field induced by molecular adsorption at the surface of the top electrode, and contribute to highlight the potential use of ferroelectric thin films as gas sensors. (C) 2011 Elsevier B.V. All rights reserved.