NaNiO2 exhibits a layered structure made of NiO2 slabs between which Na+ ions are inserted in octahedral environment. Trivalent nickel ions in the low spin state (t(2)(6)e(1)) exhibits a Jahn-Teller effect which leads to a distortion of the NiO6 and hence to a macroscopic lattice distortion. The partial substitution of trivalent cobalt for nickel (NaNi1-yCoyO2) allows to suppress the structural distortion for y greater than or equal to 0.22. This behavior results from the competition between the stabilization of Ni3+ ions in distorted environment and the strong stabilization of the isotropic Co3+ ions in octahedral environment.