The mechanical properties of superalloys are significantly enhanced by the presence of intermetallic ordered gamma' phase with L1₂ structure. They exist coherently embedded in the matrix of the superalloy and high volume fractions of up to 75% can be adjusted. While in Ni-based superalloys this is nearly exclusively the gamma'-Ni₃(Al,Ti) phase a multitude of possible phases with L1₂ structure are currently under investigation in Co-based superalloys. Development goals are an increased stability of those phases, improved mechanical strength and the avoidance of too high amounts of heavy alloying elements as e.g. W. The current work presents density functional theory calculations of gamma' ((1-x)CoxNi)3((1-y)AlyTi) phase with x and y varying between 0 and 1. The effect of the composition on the formation energies as well as electronic structures and other properties including elastic constants is investigated. One main result is that the most stable phase tends to be Ni- and Ti-rich. This result is also in agreement with experimental investigations of such alloys.