This study investigates the dependence of Ag resistivity on film thickness during temperature ramping as a means to access thermal stability. In situ van der Pauw four-point probe analysis is used to determine the onset temperature; the temperature when the electrical resistivity deviates from linearity during the temperature ramp. At that point, the silver thin films become unstable due to void formation and growth during thermal annealing. The thermal stability of Ag thin films on SiO2 in a vacuum is greatest when thicknesses are greater than 85 nm. Using an Arrhenius relation in terms of onset temperature and film thickness, an activation energy (0.32±0.02 eV) for the onset of agglomeration in Ag thin films on SiO2 ramped at a rate of 0.1 °C/s is determined. This value is consistent with the activation energy for surface diffusion of silver in a vacuum, which is believed to be the dominant mechanism for agglomeration of silver thin film. © 2002 American Institute of Physics.