Changes in macrostress and microstructure accompanying interdiffusion in sputtered niobium(45nm)-tungsten(45nm) bilayers upon annealing at relatively low temperatures (T < 1000K) have been investigated using X-ray diffraction and reflectometry. Concentration profiles resulting from interdiffusion corresponding to diffusion lengths of only a few nanometers have been determined by simulation of the measured reflectivity patterns. The X-ray diffraction stress evaluation has been based on the analysis of intensity maps recorded as a function of the diffraction angle and sample tilt angle. The occurrence of interdiffusion is associated with pronounced changes of stress of opposite nature in the Nb and W sublayers.