It is becoming clear that an understanding of heterogeneous processes in the troposphere is necessary to account for trace gas concentrations. For example, recent reports of HONO emission from vegetation and other surfaces suggest that there is a heterogeneous chemical source. A number of heterogeneous reactions studied in this laboratory, including NO₂ hydrolysis and the reaction of adsorbed HNO₃ with gaseous NO, are known to be dependent on water vapor concentration and thus are likely to be dependent on the amount of water present on the reaction surface. To fully understand these reactions, the amount of water on the surfaces and how it is distributed must be understood. Here, we report the uptake of water on a number of environmentally relevant materials that are also commonly used in laboratory experiments. The implications for understanding heterogeneous reactions in these thin films will be discussed.