Susceptibility, high-field magnetization and submillimeter wave electron spin resonance measurements of layered quasistoichiometric Li1−xNi1+xO2 are reported and compared to isomorphic NaNiO2. A new mechanism of magnetic frustration induced by the excess Ni ions always present in the Li layers is proposed. We finally comment on the possible realization of an orbital liquid state in this controversial compound. 75.40.Cx, 75.10.Jm Since its synthesis in 1958 by Goodenough et al. [1], LiNiO 2 is a subject of continuous debate. Its ideal structure can be described as a packing of Li and NiO 2 slabs built up of edge sharing NiO 6 octahedra. Therefore, magnetic Ni and nonmagnetic Li hexagonal planes alternate along the ¡111¿ direction, giving rise to a quasi-2D magnetic lattice. Detailed chemistry analysis of this compound has been motivated by its potential application in rechargeable batteries; this allowed to overcome the initial disagreement between results from different groups due to the sensitivity of the physical properties to the sample preparation method. On the other hand, theoretical interest on this system comes from the interplay between different degrees of freedom : doubly degeneracy of the Ni 3+ (t 6 2g e 1 g ) orbitals and their eventual coupling to the S=1/2 spins, the effect of frustration in the triangular Ni lattice, the elusive nature of the magnetic interactions. In spite of numerous studies and significant progress on these subjects [2], the puzzling absence of both orbital and magnetic ordering, indeed clearly observed in isomorphic NaNiO 2 [3,4], remains a mystery. More recently, LiNiO 2 has been considered as the first realization of a quantum spin orbital liquid [5][6][7][8].Here we report new measurements on well characterized homogeneous Li 1−x Ni 1+x O 2 . We study three samples, one of them being, to our knowledge, the closest one to stoichiometry reported up to now, and we compare their behavior to NaNiO 2 . The overall results agree with recent theoretical development [9] concerning the decoupling of the orbital and spin degrees of freedom, particular to these frustrated Jahn-Teller (JT) systems, and the always ferromagnetic (FM) sign of the intralayer Ni-Ni magnetic interactions. They also confirm the cluster model that we have proposed [10] to describe the dependence on the concentration x of different properties. Furthermore, we can conclude that most probably stoichiometric LiNiO 2 does not exist, and that the clusters formed around the excess Ni ions on Li planes are responsible for the peculiar behavior of this system. In fact, when the intrinsic weak antiferromagnetic (AF) interaction between adjacent Ni layers, neglected in previous theoretical works [9,10], is taken into account, the effective FM coupling induced by those clusters frustrates the AF stacking of the FM Ni planes, hindering the long range 3D-magnetic ordering observed in NaNiO 2 below 20K [4].The detailed description of the synthesis conditions and of the structural characterization of our NaNiO 2 and Li 1−x...