  Co substituted Ni$_{1-x}$Co$_x$Br$_2$ (0 $\leq~x~\leq$ 1) single crystals
were synthesized using vapor transport. The physical properties of the crystals
were characterized by x-ray powder diffraction, magnetization, and specific
heat measurements. Room temperature x-ray powder diffraction data indicate a
change from the CdCl$_2$ structure type to the CdI$_2$ structure type occurs
within 0.56 $<~x~<$ 0.76. NiBr$_2$ has a commensurate antiferromagnetic phase
below $T_{\rm N} \approx$ 46 K and an incommensurate magnetic ground state
below $T_{\rm IC} \approx$ 20 K. Both magnetic transitions are affected by
cobalt substitution, and the incommensurate phase transition is present up to
at least $x$ = 0.56. The evolution of magnetism has been studied as a function
of cobalt content and is summarized in the temperature-composition phase
diagram.
