Crystals of R 2 BaNiO 5 (R= Y, Er) have been grown, and their structures have been established by single-crystal x-ray diffraction. Both compounds crystallize in the Nd 2 BaNiO 5 structure type, with one-dimensional chains of vertex-sharing NiO 6 octahedra in the direction of the a axis. These octahedra show an unusual twofold distortion: The Ni-O distances to the two axial oxygen atoms are considerably shorter, 0.3 Å, than those to the four equatorial oxygens, and these oxygens are distorted from the right angles of a regular octahedron to 79.0 (2) or 77.7 (6), respectively. As a result of this, Ni-O (axial)-Ni distances are very short, 3.76 and 3.75 Å for R= Y and Er, respectively. X-ray powder diffraction data and the results of magnetic measurements for both oxides are given. The structural features mentioned elucidate why Ni 2+ ions in polycrystalline Y 2 BaNiO 5 behave as a monodimensional system in which they become antiferromagnetically ordered below 300 K. Besides that, the ferromagnetic interactions that operate below 40 K can be due to tridimensional interchain interactions and/or the presence of ferromagnetic impurities. The estimated Néel temperature for Y 2 BaNiO 5, higher than that reported for Y 2 BaCuO 5, is explained by the promotion of the superexchange Ni-O-Ni interactions along the chains of flattened NiO 6 octahedra sharing corners. In Er 2 BaNiO 5 both effects are masked by the strong paramagnetic signal of Er 3+, and a maximum observed at 15.6 K for the susceptibility is attributed to tridimensional ordering of the Er 3+ cations.