CuSn is an important intermetallic compound used in lead-free soldering and anode materials for Li-ion batteries. Cu Sn exists in at least two crystal structures, with a transformation from hexagonal η-CuSn at temperatures higher than ∼186 °C to monoclinic η′-CuSn at lower temperatures. Using variable temperature synchrotron X-ray diffraction analysis, the kinetics of the η-η′ transformation are investigated and a time-temperature transformation diagram is partially developed. The significance of the results in terms of phase and dimensional stability in solder joints are discussed.