The aggregation of three cationic dyes (CD), crystal violet (CV), Nile blue (NB) and rhodamine B (RB) in aqueous solution was studied by visible absorption spectrophotometry and compared with methylene blue (MB). The distribution of the dye species (monomers, dimers, trimers, and tetramers) in aqueous solutions with different concentrations of dye was calculated using equilibrium stepwise aggregation constants Kn
. These cationic dyes were intercalated into montmorillonite (SAz-1) and its reduced charge form (RC-SAz(210)) prepared by heating lithium montmorillonite (Li/SAz-1) at 210ºC. The fluorescence of fully saturated CD/SAz and low-CD loaded CD/RC-SAz(210) complexes was studied. Visible absorption spectra of CD aqueous solutions and visible absorption spectra and X-ray diffraction patterns (d
001) of the CD/SAz and CD/RC-SAz( 210) solid complexes were obtained and evaluated. Large fluorescence intensities were found for CV/RC-SAz(210) and NB/RC-SAz(210) complexes in the same way as for the complex of methylene blue with reduced-charge montmorillonite MB/RCM(210) described previously.