Abstract: Nickel nitrate solution is potentiometrically titrated (a) with montmorillonite, (b) without montmorillonite, in the nitrate solution. X-ray powder diffraction and chemical analyses are made of the products formed by the clay at various stages of titration. The pH values of the titration curve (a) are lower than those of curve (b) until the brucite-like hydroxy-nickel interlayer is more or less complete. The resulting product then closely resembles a chlorite. The preferential precipitation of hydroxide in the interlayer region is explained in terms of the acidic character of montmorillonite interlayers.