Although the role of multiphase chemistry that occurs between gases and surfaces in the atmosphere, such as aerosol particles and cloud droplets, is now well recognized, the comparable chemistry that occurs with indoor surfaces is not as well characterized. This seminar will address the general topic focusing on the fundamental chemistry that results. Both results from laboratory experiments as well as measurements from genuine indoor environments will be presented. Topics include the nature of the oxidation that results when highly unsaturated condensed-phase organics, such as those in skin oil, are oxidized by ozone leading to the formation of Criegee radicals and higher-order oxidation products. As well, interesting chemistry results when a floor is washed with sodium hypochlorite solution (i.e. bleach), leading to the heterogeneous uptake of HOCl vapour by surfaces throughout a room. This chemistry likely proceeds via HOCl uptake across carbon-carbon double bonds. Finally, although the formation of HONO via interactions of NO2 with wet surfaces is now well recognized, new measurements in a residential home allow us to examine the detailed mechanism that sustains HONO at levels substantially higher indoors than outdoors. Photolysis of HONO is one of the major OH sources in indoor environments.