Magnetic studies have been carried out down to liquid-nitrogen temperatures on NN′-ethylenebis(salicylideneiminato)cobalt(II) and many of its ring-substituted derivatives. No relationship between the detailed magnetic properties and oxygen-carrying ability of this type of compound has emerged. A variety of magnetic behavior is exhibited by this type of compound. The majority (the unsubstituted, 5-Me-, 5-Br-, 5-MeO-, 4-Cl-, 3-MeO-, and 3-EtO-substituted compounds) are low-spin, with magnetic moments which are in the range 2·1–2·9 B.M. and almost invariant with temperature, as expected for planar cobalt(II) compounds. In addition to anhydrous, magnetically normal forms, the 3-MeO- and 3-EtO-substituted compounds form monohydrates in which, as judged from the accompanying changes in magnetic, spectral, and oxygen-carrying properties, the water molecule is co-ordinated to give a tetragonal pyramidal structure. The 3-MeO hydrate is an example of the rare high-spin pentaco-ordinate type of cobalt(II) compound. The magnetic behaviour of the 3-EtO hydrate can be explained in terms of a high-spin–low-spin equilibrium. The sharp changes in slope found by earlier workers at about 200°K in the curves of reciprocal susceptibility against temperature of the monopyridine complex and the inactive form of the parent complex have not been confirmed. These, and the 5-nitro- and 3,5-dinitro-substituted complexes, have low-spin magnetic moments which decrease with temperature to values expected for octahedral, low-spin cobalt(II) complexes. This is considered to be due to the effect of axial ligands, i.e., pyridine, or oxygen atoms of adjacent molecules.