The magnetic behavior found for R 2BaNiO 5 ( R = Sm, Gd, Yb and Lu) compounds has been explained by taking into account the two structural types in which they crystallize. For both Sm and Gd isostructural oxides above room temperature, the nickel ions are 1D-antiferromagnetically ordered along the chains of octahedra characteristic of the Nd 2BaNiO 5 structure type. Below 30 K both rare earth and nickel sublattices are simultaneously 3D-ordered. In the case of Yb and Lu oxides, belonging to the Sm 2BaCuO 5 structure type, the existence of isolated distorted square pyramids (NiO 5) justifies the assumption that antiferromagnetic interactions in the nickel sublattice are not operative until the temperature is as low as 10 K. Moreover, no evidence of order in the ytterbium compound has been found down to a temperature of 4.2 K. On the other hand, high-resolution electron microscopy reveals that both types of material are well ordered, without any local structural defects.