In this work, the results for the modification of a montmorillonite-type clay with a polycationic biopolymer – chitosan- are presented, along with the evaluation of the effect of contact time between clay and modifiying agent on the effectiveness of the modification treatment. Modified clays were prepared using high molecular weight chitosan, following a specially designed methodology and applying stirring times of 3, 6 and 12 hours. The effectiveness of the modification process was evaluated by measuring the increase in the interlayer spacing of the clay, and the detection of characteristic functional groups from the chitosan in the structure of the clay. From the XRD spectra it was observed that the interlayer spacing increases with the increase in contact time between clay and modifier, leading to a pronounced shift and intensity decrease of the characteristic peak, which corresponds to the (d 001 ) plane of montmorillonite. The existence of active groups from chitosan in the modified clay structure was evident from the presence in the Infrared Spectra of absorption bands corresponding to CH, NH 3 + , CH 2 y NH 2 .