Structural changes in a cobalt-based alloy (Co-31Cr-12.5W-2.2C where the composition is in approximate weight per cent; Stoody 3) as a result of high fluence nitrogen or titanium ion implantations were investigated via transmission electron microscopy and selected area diffraction examinations of unimplanted and implanted foils. The alloy microstructure was found to consist of several morphologies of single-crystal carbides (CrCoW solutions) in a cobalt-rich f.c.c. matrix phase of high planar defect density. Titanium implantation (5 × 10 17 Ti + cm −2 at 190 keV) produced a surface layer with an amorphous matrix phase and recrystallized carbides, while nitrogen implantation (4 × 10 17 N + cm −2 at 50 keV) greatly increased the planar fault density in the matrix phase. Previously reported effects of titanium and nitrogen implantations on the tribological behavior of the alloy are discussed in terms of the results of these investigations.