The martensitic transformation on thermal cycling of the polycrystalline Zr(Cu0.5Co0.25Ni0.25) high temperature shape memory alloy was examined using in situ synchrotron X-ray diffraction. A single step B2 ↔ B19′ transformation was observed with a thermal hysteresis of 161 °C and a martensite finish temperature of 294 °C. Large anisotropic lattice constrictions and dilations, up to 1.3%, were observed during the course of transformation in the B19′ martensite, but corresponding changes in the B2 austenite were not observed. The coefficients of thermal expansion in both phases were found to be quite large, 33×10-6°C-1 in B2 and 39, 88 and 29 × 10−6 °C−1 for aB19′, bB19′ and cB19′ respectively. Possible mechanisms governing the transformation behaviour are discussed.