The objective of this work was to examine the effects of heat treatment on the mechanical properties and microstructure of two sets of high cobalt martensitic precipitation strengthened stainless steels which can achieve yield strengths of about 1650 MPa. The first series of alloys consisted of three compositions. The three alloys contained (in wt. %) about 0.005 carbon, 14 chromium, 5 molybdenum and 1.5 nickel and differed only in cobalt contents which were 18, 19.5 and 21 wt. %. The second series of alloys also consisted of three compositions. These three alloys all contained (in wt. %) 0.025 carbon, 14 chromium, 5 molybdenum, 1.5 nickel and 0.025 titanium and differed only in cobalt content which were 16, 17 and 18 wt. %. The carbon and titanium additions to the second series of alloys were made with the objective of gettering the sulfur as particles of titanium carbosulfides because particles of titanium carbosulfide are much more resistant to void nucleation than other types of sulfides and this greater resistance void nucleation has been to improve the toughness of other ultra-high strength steels. In this work the effects of tempering temperature, refrigeration after cooling from the austenitizing temperature, the rate of cooling from the austenitizing temperature, austenitizing temperature and of double austenitizing heat treatments on the Charpy impact energy, the tensile properties, fracture modes and microstructure were evaluated.