The adsorption of the monovalent anionic dye alizarinate onto Na- and Al-montmorillonite was carried out by adding the dye into aqueous clay suspensions. Electronic spectra of aqueous suspensions and of air-dried dye-clay complexes were studied. Na-montmorillonite adsorbed only part of the added dye. With total amount of alizarinate up to 5 mmol dye per 100 g clay the adsorption of the dye takes place on the broken bonds, leading to peptization of the clay. Al-montmorillonite adsorbed alizarinate completely up to 10 mmol per 100 g clay. Above this loading there was a partition of the dye between the clay and the supernatant. The maximum adsorption for Na- and Al-clay was 4 and 25 mmol dye per 100 g clay, respectively. Absorption bands in the spectrum of Al-montmorillonite suspensions (488-504 nm) appear at longer wavelengths than in the spectrum of air-dried Al-montmorillonite (415-455 nm). Thermo-X-ray study of these clay-alizarinate complexes suggests that the organic compound was located in the interlayer space in Al-montmorillonite but was not located there in Na-montmorillonite. In Al-montmorillonite alizarinate formed a coordination complex with exchangeable Al3+. In Na-montmorillonite it formed bonds with Al exposed on the broken-bonds sites. (C) 2003 Elsevier Inc. All rights reserved.