Electron spin resonance (ESR) spectra of Cu2+-hexadecylpyridinium (HDP) montmorillonites were investigated as a function of HDP+ content and the hydration state of Cu2+ at relative humidities of p/p0 = 0.52-8 × 10-7 at 298°K. The symmetry of the Cu2+ ESR spectra and the intensity of the ESR signal increased upon dehydration of the complex. The HDP+ cation caused an increase in the hydration state of Cu2+ at a given p/p0 and an increase in the covalency of the Cu-O bond.