Complexes of Na-montmorillonite with three amino acids: lysine, arginine and histidine were prepared at different pH values. The basal spacings of the complexes were assessed by X-ray diffractometry at controlled condition of relative humidity. For those complexes, the basal spacing unchanged with relative humidity with the exception of those at high pH (10 and 13). For these samples the change in the basal spacing with relative humidity was the same as that seen for montmorillonite alone. At high pH (>10) lysine, arginine and histidine were apparently not intercalated into montmorillonite. These observations may be explained in terms of the pH-dependent charge characteristics (dissociation) of the amino acids used. Lysine, arginine and histidine have an amino group in both the main and side chain of the molecules. At low pH, both amino groups will be positively charged. At high pH, amino groups will hardly have no charge, though a carboxyl group will almost charged negatively. Since interlayer complex formation is dependent on molecular charge, the basal spacing of montmorillonite-amino acid complexes is sensitive to variations in relative humidity.