The compression strength of an Ir-15at% Nb alloy at 1473 and 2073 K was investigated. A coherent two-phase fcc L1(2) structure was found in the Ir-15Nb alloy. The L1(2) precipitate morphology depended on the heat treatment. Cuboidal L1(2) precipitates with a size of 100 nm and a plate-like fcc phase inside ( type A) were found after heat treatment at 1773 K. The plate-like fcc phase disappeared after heat treatment at 2073 K for 24 h, and only a cuboidal L1(2) phase remained ( type B). Coarse rectangular L1(2) precipitates with a length of 400 nm ( type C) were found after heat treatment at 2073 K for 168 h. The influence of L12 precipitate morphology on the high-temperature strength and dislocation structure was investigated after the compression test. A bypass mechanism in which dislocations spread in the narrow fcc phase was dominant in the type A and B structures during deformation at 1473 K. In the type C structure, bypassing of precipitates was found to be dominant. At 2073 K, deformation by a shearing mechanism was dominant in the type A and B structures, while deformation by a bypassing mechanism was dominant in the type C structure. When the precipitate size was large and the fcc channel width was wide in the type B structure, a bypassing mechanism was dominant. The deformation mechanisms are discussed in terms of the precipitate morphologies. (C) 2003 Kluwer Academic Publishers.