Metastable face-centred cubic (fcc) Fe precipitate particles produced by ageing of a Cu-Fe alloy with up to about 2 wt % 5 Fe are fully coherent with the Cu matrix when they are less than about 75 nm in diameter [1]. The coherent particles do not transform into stable body-centred cubic (b c c) Fe even if the alloy is cooled to liquid-helium temperature [2]. However, the f c c particles martensitically transform into internally twinned b cc structure by plastic deformation of the alloy [3-5] or by partial or complete removal of the surrounding Cu matrix [3, 4, 6]. Loss of full coherency in larger particles is not sufficient to induce the transformation, at least until the particles coarsen in size to several hundred nm [1]. However, ternary addition of Co to a C u F e alloy tends to destabilize the fc c particles and, thus, the fc c-to-b c c martensitic transformation in F e C o particles can occur by simple cooling without the introduction of plastic deformation [7, 8]. The transformed particles are also internally twinned. This study investigated further details on the transformation characteristics during cooling of F e C o particles in C u F e C o alloys with various Co contents. 98 wt% Cu-2 wt% Fel_x Cox alloy ingots with x = 0, 0.2, 0.3, 0.35, 0.4, 0.5 and 0.6 were prepared by melting 99.99% purity Cu, 99.9% Fe and 99.99% Co in a high-frequency induction furnace. To promote homogenization, the ingots were turned upside down, remelted several times and then maintained at 1273 K for 1 week in a vacuum furnace. It was confirmed by chemical analysis that the compositions of the alloys were essentially the same as the initial compositions. Hereafter, the alloys with x = 0-0.6 will be denoted A-0-A-6. Sheet specimens cut from the ingots were aged at 923 K for 6 or 96 h after a solution treatment at 1273 K for 2 h. After the ageing the specimens were slowly cooled to given temperatures above 77 K to prevent the introduction of thermal stresses. The 6 h ageing produced spherical f cc particles with average diameter 21 nm, and the 96 h ageing gave the same but an average diameter of 52 rim. The particle size was essentially independent of the compositions of the alloys. After the cooling the specimens were annealed at 873 K for 1 h and quenched into water, except for the specimens for structural examination of particles before and after the martensitic transfor-