We investigate in the framework of density functional theory the structural and electronic properties of stoichiometric L1₁ ordered transition metal alloys of Pt and the 3d transition metals Mn, Fe, Co, Ni and Cu. A marked dependence of the energy difference between L1₁ and L1₀ structure on the valence electron concentration is encountered, with the L1₁ order being the preferred structure for CuPt, whereas the other alloys favor the L1₀ arrangement. The changes of the electronic density of states on composition are well represented within a rigid-band-picture, while the transition from L1₀ to L1₁ order is accompanied by a characteristic redistribution of the minority spin states around the Fermi level.