Hydroxyhectorites containing Ni(II) in lattice sites (Mg-NiHect) and in ion-exchange sites (NiMg-Hect) are synthesized and characterized by electron spin resonance (ESR) spectroscopy after gamma-irradiation at 77 K to generate Ni(I). Two Ni(I) species, species A (g(parallel to) = 2.51 and g(perpendicular to) 2.09) and species B (g(parallel to) 2.24 and g(perpendicular to) = 2.11), are identified in the ESR spectrum of Mg-NiHect. Only one Ni(I) species (g(perpendicular to) = 2.09) is identified in NiMg-Hect. After annealing at room temperature, the intensity of the ESR spectra of Ni(I) species in NiMg-Hect increased substantially with a concomitant decrease of the ESR intensity of trapped hydrogen atoms. The intensity of Ni(I) species B in Mg-NiHect did not increase after annealing at room temperature, but the intensity of Ni(I) species A increased slightly. The Ni(I) species in NiMg-Hect was not stable after adsorption of D2O or CD3OH and annealing at room temperature. But Ni(I) species B in Mg-NiHect was stable after adsorption of D2O or CD3OH and annealing at room temperature, while Ni(I) species A was unstable. According to these ESR characteristics and to chemical analyses before and after treatment with ethylenediaminetetraacetic acid/sodium acetate, the Ni(I) sites are assigned as follows. The Ni(I) site in NiMg-Hect is assigned to a nonframework ion-exchange site in the interlayer space. Ni(I) species A in Mg-NiHect is assigned to Ni(I) in an edge framework site in the octahedral sheet of a clay layer, and Ni(I) species B in Mg-NiHect is assigned to Ni(I) in an interior octahedral framework site in the octahedral sheet of a clay layer.