It has been suggested (Hill, Pratt & Williams 1962) that the catalytic activity of ‘ annular ’ complexes (in which four coordination positions are occupied by a planar or semi-planar ring system as in vitamin B12 and the iron porphyrins) may be due to the presence of an interaction between two or more reactive sites in the same molecule. In studying the chemistry of B12 and other corrinoids, therefore, we are investigating the interaction between the conjugated ring, the cobalt ion and the two axial ligands by a variety of techniques including absorption and reflexion spectroscopy, circular dichroism (c.d.), proton magnetic resonance (p.m.r.), electron spin resonance (e.s.r.) and controlled potential reduction (c.p.r.). We are greatly indebted to Dr E. Lester Smith, F. R. S., who has provided many of the compounds.