Interdiffusion and intermetallic compound formation in Cu-Sn thin film couples have been investigated by X-rays using a Seemann-Bohlin diffractometer. The films were prepared by consecutive evaporation at room temperature on fused quartz substrates and subsequently annealed between - 2 and 100°C. The η′-phase (Cu6Sn5), which is ordered, was found to grow at all temperatures. In contrast the ordered ε-phase (Cu3Sn) was found only in those specimens that had been annealed above 60°C. The formation of the η′-phase is interpreted in terms of interstitial diffusion of Cu into Sn at low temperatures. In specimens maintained at room temperature the unreactecl Sn and Cu layers were inferred to be in compression and tension, respectively. Tin whiskers were observed to grow spontaneously at room temperature from the Sn surface of the Cu-Sn bimetallic films, but not from Sn films without the Cu underlaver. The driving force for whisker growth is attributed to interdiffusion and reaction that occur in the bimetallic films.