New (R_{2} \mathrm{BaCoO}{5}) oxides ( (R=) rare earth) have been prepared in Ar flow, and the (X)-ray diffraction data reveal that they crystallize in the (\mathrm{Nd}{2} \mathrm{BaNiO}{5}) structural type (space group Immm). The existence of dimorphism has been shown for (R=\mathrm{Dy}, \mathrm{Ho}, \mathrm{Er}), and Tm. A comparative study of the influence of the rare earth size on the structural parameters is presented for this family of compounds. The relative stability of these oxides is discussed from the variation of the so-called global instability index (GII) calculated using the bond valence method. The GII values around 0.2 v.u. are indicative of the high stress present in this structure. The magnetic properties of these oxides have been studied from magnetic susceptibility measurements and one-dimensional antiferromagnetic correlations in the (\mathrm{Co}^{+2}) sublattice are presented around the room temperature, while three-dimensional antiferromagnetic interactions in both (\mathrm{R}^{+3}) and (\mathrm{Co}^{+2}) sublattices have been observed at lower temperatures (\left(T{\text {Neel }}<40 \mathrm{~K}\right.) ) in this family of oxides. This behavior is explained considering the crystal structure of these compounds and the electronic states of the (\mathrm{Co}^{+2}) and (\mathrm{R}^{+3}) cations. 1994 Academic Press, Inc.