Co-substituted Ni1-xCoxBr2 (0≤x≤1) single crystals were synthesized using vapor transport. As a result, 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 that a change from the CdCl2 structure type to the CdI2 structure type occurs within 0.56<x<0.76. NiBr2 has a commensurate antiferromagnetic phase below TN ≈ 46 K and an incommensurate magnetic ground state below TIC ≈ 20 K. Both magnetic transitions are affected by cobalt substitution, and the incommensurate phase transition is present up to at least x=0.56. Here, the evolution of magnetism has been studied as a function of cobalt content and is summarized in the temperature-composition phase diagram.