The adsorption of tris (1, 10-phenanthroline) copper (II)([Cu (phen) ₃] ²⁺) by a clay mineral has been studied with electron spin resonance (ESR) measurements. The chelate, tris (1, 10-phenanthroline) copper (II)([Cu (phen) ₃] ²⁺), is co-adsorbed with a large excess of racemic or enantiomeric [Ru (phen) ₃] ²⁺. The spectra of a freezedried sample of a clay-chelate adduct at 77k show that there are two kinds of adsorption states: the one (denoted by state A) is characterized by the parameters of g_II=2.263-2.275 and A_II=151-160G and the other (denoted by state B) by g_II=2.213-2.229 and A_II=150-186G. States A and B are postulated to be the adsorptions on an external surface and in the interlayer space of a clay mineral, respectively. The effects of chelate loading and swelling with organic solvents on the relative abundances of these two states are interpreted on the basis of the above assumption.