Differentially pumped X-ray photoelectron spectroscopy has been used to study the surface composition of sulfuric acid (H2SO4) and sulfuric acid/water mixtures in equilibrium with various atmospheric species. Nitric acid uptake in sulfuric acid solutions was observed below approximate to 220K. Compared to the theoretically predicted bulk composition, a significant enrichment of nitrogen was observed at the interface. The uptake of background water in sulfuric acid below 200 K was responsible for the reversible uptake of CO2, moderated through the formation of carbonic acid (H2CO3). For CH2O, Cl-2, NO, and NO2, no observable surface species were observed over the temperature range studied (approximate to 180-298 K).