Technology of obtaining montmorillonite modified by polyionenes is developed. Macromolecular polymer intercalation of a quaternary ammonium salt of montmorillonite intercrystalline space is shown to be accompanied with increased interlayer distances from 1.08 nm to 1.67 nm. The technique of synthesis of montmorillonite modified by polyionenes is suggested. Optimal conditions for sorption of polyionenes molecules with montmorillonite are found to be: the concentration of aqueous dispersion of montmorillonite is 1 %, the temperature of the reaction medium is 40 °C, the montmorillonite-polyionene ratio is 3 : 1, the processing time is 24 hours. The mechanism of montmorillonite modification is suggested to involve the next steps: connection of organic cations to montmorillonite surface determined by attachment of organic cations to exchange position during ion-exchange adsorption and adsorption of organic cations with acid sylanol groups, i.e. torn bonds on crystal faces. These processes are shown to result in more perfect structure by organic cations adsorption with acid sylanol groups (torn bonds on crystal faces).