This work presents the changing abundance of surface functional groups (SFG) on polystyrene (PS) upon weathering within one or a few molecular monolayers from a molecular point of view. PS particles were aged by exposing them to a gas flow of typically (5%) O3 in air (PSO3), UV radiation using a solar simulator under controlled conditions in the laboratory (PSSS) and by exposing PS samples to the water/air interface immerged in a freshwater lake for 2 months (PSL). The chemical composition of the interface of weathered, compared to pristine (virgin or PSV) material was established using a titration technique that probed the chemical composition of the interface of the polymer. The main conclusions of this exploratory study are: (a) The interface of PS changes significantly compared to ATR FTIR spectra that do not show additional absorptions in the mid-IR spectrum over a penetration depth of more than hundred monolayers at 10 mm; (b) The average surface functionalization of the gas-solid interface, corresponding to the sum of all examined types of SFG, increases from 20% of a monolayer for PSV to 40, 50 and 84% for PSL, PSO3 and PSSS, respectively; (c) in all cases the most important SFG was surface -OH ranging from 11.2 to 64% for PSV and PSSS, respectively; (d) each PS sample shows a characteristic SFG pattern or fingerprint using several probe gases; (e) O3 interaction led to interface acidification; (f) UV treatment leads to the highest degree of surface -OH functionalization compared to PSO3 and PSL. The accumulation of SFG’s renders the interface more reactive towards adsorption of probe gases.