Time-resolved Fourier transform infrared (FTIR) spectroscopy is combined with dynamic mechanical measurements in order to unravel the interplay between the amorphous chains and nanocrystals in spider silk under conditions of selective swelling. The sample remains humid after supercontraction, identifying in this way the role of the softened amorphous phase. For the first time, the amount of prestress in the fibers can be determined by direct comparison between microscopic and macroscopic stress during humidity increase. Nanocrystal stress decreases by similar to 120 MPa, even though the external stress increases at the same time. The current findings suggest that water and its interplay with pre-stress determine the relative contribution of crystals and amorphous matrix to the mechanical properties. Increased water content and release of pre-stress deteriorate the macroscopic mechanical properties, as the system becomes viscoelastic and loses its stiffness. (C) 2011 Elsevier Ltd. All rights reserved.