Kinetics of formation of Cu6Su5 between Cu and Sn thin films at room temperature and of Cu3Sn between Cu6Sn5 and Cu at temperatures from 115° to 150°'C were measured by Rutherford backscattering spectroscopy and glancing-incidence X-ray diffraction. The growth of Cu6Sn5; showed a linear rate and the reduction of Cu6Sn5 due to the growth of Cu3Sn showed a parabolic rate with an activation energy of 0.99 eV. A flash of W film was deposited between Cu and Sn as diffusion markers during their room temperature reaction. Marker displacement measured by the backscattering technique showed that Cu is the dominant diffusing species in forming Cu6,Sn5. By annealing a Cu3Sn/Cu sample at 640°C for 20 min and by cooling (not a rapid quenching) it to room temperature, we found that the high temperature phase Cu4Sn can be made metastable at room temperature.