The effects of cold-working and age-hardening on the microstructure of Elgiloy (composition in wt %, 40 Co, 20 Cr, 15 Ni, 7 Mo, 2 Mn, 0.1 C, balance Fe) have been established. 36% and 72% cold reduction of the solution treated (ST) alloy (f c c) produced a network of thin f c c deformation twins and h c pe-martensite platelets. The subsequent age-hardening of the cold-worked strip noted at 500° C was attributed to the formation of additional e-phase (via theαf c c ⇌eh c p transformation), whereas the ST condition did not harden significantly at this temperature. In contrast, ageing the ST condition at 800° C for periods >1000 h caused an increase in hardness due to the formation of a coarse Mo-Co-Cr intermetallic compound together with a smaller amount of M23C6 carbide, while age softening of the cold-worked strip occurred at this temperature as a result of recovery and dissolution of the deformation inducede-phase.