A sensitive method has been developed for the measurement of mechanical stresses in compliant planar samples originating from changes of their solvent content or temperature. The samples are mechanically conjoined with a silicon wafer. Thus, an alteration of the sample stress causes a spatial displacement of the wafer which is detected capacitively. The method is applied to the measurement of stresses in both acetone-dried and air-dried chrome-tanned leather which has been exposed to a stepwise modulation of ambient humidity. The stress development in the sample can be well characterized by a first-order exponential decay. The mean moisture content exhibits a similar time dependence. However, the relation between stress development and moisture content differs for swelling and shrinkage. This behavior is explained on the basis of a novel two-capillary model. Moreover, the response of the sample to the alteration of ambient humidity is found to be related to the structure of its collagen fiber network. The tight structure of air-dried leather with a poor isolation of fibrils yields much higher stresses than acetone-dried leather.