The surface basicity of the oxygen planes of four Na-smectites and one Na-vermiculite was studied by visible spectroscopy of clay suspensions saturated with the metachromic cationic dye acridine orange. A metachromic hand in the spectrum is an indication of pi interactions in which the cationic dye is involved. Curves describing the wavelength of the metachromic band in the presence of clay minerals as a function of the degree of saturation showed three regions. X-ray diffractions proved that in the first region monomeric acridine orange cations were located in the interlayer space with the aromatic sings lying parallel to the silicate layers. Thus, metachromasy in the first region was due to pi interactions in which atoms from the O-planes donate lone-pair electrons to pi antibonding orbitals of the organic cations. From the location of the metachromic band it was concluded that the basic strength of the O-planes decreased in the order : beidellite > vermiculite > montmorillonite > saponite > laponite.