The effect of boron addition, introduced in different amounts (0.01, 1.48%) in heat-resistant cobalt-casting alloys of the MAR-M509 type, on the kinetics of precipitation of secondary carbides Cr23C6 is investigated during annealing at 975 °C for 21 h. Structure analysis of cobalt alloys tested in the as-cast condition and after annealing is also presented. It has been found that boron significantly shortens the incubation time for forming nuclei carbides of Cr23C6. Boron at 0.09% causes size reduction of primary phases, whereas higher boron additions create additional boride phases of the MB type of high tungsten content. Dilatometer curves for the precipitation of secondary carbides Cr23C6 in cobalt alloys of the MAR-M509 type have been described using Wert's empirical equation,W t=kt n .