Thin polymer films have found widespread application in silicon-based microelectronic devices and micromechanical systems due tc, their unique material properties. When exposed to ambient humidity polymers tend to expand due to the uptake of moisture in the film. In most microelectronic applications. this characteristic is rather undesirable due to associated reliability problems. We have investigated humidity-dependent mass uptake and expansion for a range of polymers at room temperature and have found that the isotherms are generally nonlinear. Based on the obtained relationships. the polymer process can be optimized in viers of reduced mass uptake and reduced mechanical stress in the polymer. Results are given for polyimide PI2540. Both mass uptake and swelling can be approximated by a linear humidity dependence with an error of about 10%. Based on these results, the empirical quantities swelling ratio and stress coupling factor are defined, and linearized sorption parameters are discussed.