In the two isomorphous title compounds, viz. tris[2,2'-bi(4,5-dihydro-1,3-oxazole)-kappa(2)N,N']copper(II) diperchlorate, [Cu(C6H8N2O2)3](ClO4)2, (I), and tris[2,2'-bi(4,5-dihydro-1,3-oxazole)-kappa2N,N']nickel(II) diperchlorate, [Ni(C6H8N2O2)3](ClO4)2, (II), the MII ions each have a distorted octahedral coordination geometry formed via six N atoms from three 2,2'-bioxazoline ligands. For each ligand, the two five-membered rings are nearly coplanar. It is noteworthy that the Jahn-Teller effect is stronger in (I) than in (II). The three-dimensional supramolecular structures of (I) and (II) are formed via weak hydrogen-bonding interactions between O atoms from perchlorate anions and H atoms from 2,2'-bioxazoline ligands.