X-ray diffraction has been used to measure the strain relaxation in passivated Al–0.5% Cu lines at 200 °C after cooling directly from an anneal at the passivation deposition temperature of 380 °C. Fits to the measured X, Y, and Z components of strain are summed to obtain the hydrostatic component, which exhibits a decay over time. Three mechanisms are considered to explain the decay of the hydrostatic strain in the metal line: Cu precipitation from the solid solution, the presence and growth of voids in the lines, and time-dependent deformation of the passivation. Calculations of the effect of Cu precipitation from the solid solution demonstrate that it plays an insignificant role in the relaxation. A high-voltage scanning electron microscope is used to image the presence and growth of voids through the passivation. The time scale of the growth of stress-induced voids is not the same as the hydrostatic relaxation, indicating that voiding is not solely responsible for the observed relaxation. The relaxation of the line is modeled using a time-dependent finite element model, allowing elastic compliance of the passivation. The magnitude of the calculated relaxation agrees with the measurements. It is suggested that a combination of voiding and passivation compliance is responsible for the measured hydrostatic strain relaxation in the metal line.