X-ray photoelectron spectroscopy (XPS) was used to determine surface chemical composition of atmospheric particles before and after immersion in saline and bronchoalveolar lavage fluid (BALF). Atmospheric particulate matter (PM2.5) was collected on PTFE filters from clean air, outdoor urban and smoke-filled indoor sites. Low particle loads were present from the clean air site and the particle surface consisted of carbon, Cl- and oxide species. An increase in particle load was observed for the outdoor urban site with C-(C, H) compounds dominating the particle surface. There was a significant contribution from C-(O, N) and C=O/COO-functionalities as well as oxides with traces of NO3-, NH4+, amide, SiC and SO42- present. A further increase in particle load was observed for the smoke-filled indoor site. The surface consisted of 97% C-(C, H) compounds with traces of oxide, amide and SiO2. The particle load was reduced in all cases after immersion in saline mainly due to removal of loosely bound particles, especially for carbon. Changes in surface composition of the particles were also observed with removal of Cl- from the clean air site, NO3-, NH4+, amide, So(4)(2-) and SiC from the urban air site and SiO2 from the indoor smoke site; these species were deemed to be bio-available. Similar results were obtained after immersion in BALE However, there was evidence of interaction of constituents from BALF with particles collected from the outdoor urban and indoor smoke sites. A strong amide signal was observed on particles remaining on the filter after immersion in BALF suggesting that possibly proteins or other N-containing biomolecular species from BALF were adsorbed on the surface of these particles. The surface concentrations of amide, oxide, C-(O, N) and C=O/COO- varied between outdoor urban and indoor smoke particles after immersion in BALE This infers that a different interaction is occurring between BALF constituents and outdoor urban and indoor smoke particles, respectively.