Adsorption of a large cationic complex on the surface of a montmorillonite colloid results in flocculation of the colloid. This destabilization of the colloid does not occur when the cationic complex has been modified to contain a side-chain terminating in an anionic group (i.e., the complex is bipolar). The influence of such a bipolar complex on the particle sizes in montmorillonite colloids was examined using a polypyridylosmium(II) complex. Laser Doppler velocimetry was employed to determine the particle-size distributions in montmorillonite colloids containing various concentrations of the osmium complex and 1.00 g L-1 of clay. At loadings below the cation-exchange capacity of the clay, the particle-size distribution was independent of the total concentration of the bipolar osmium(II) complex. Under these conditions, at least 98% of the complex was adsorbed on the clay.