Abstract The adsorption of polysaccharides by clays has been widely studied, but its consequences on clay fabric and clay aggregation have received little attention. Therefore, the interactions between St. Austell kaolinite and Wyoming montmorillonite and the microbial exopolysaccharides, dextran and scleroglucan, were investigated. Adsorption isotherms were determined by an anthron procedure, and the fabrics of the complexes were examined by X-ray powder diffraction (XRD), low-angle X-ray scattering (LAXS), and scanning electron microscopy (SEM). The wet-state organization of clay particles was preserved as much as possible. The aggregative effect of the polysaccharides was investigated using a wet sieving test. The adsorption of scleroglucan was much higher than that of dextran. Three factors related to the structure of the polysaccharides are proposed to explain this difference, namely, the influence of corn-formation, apparent molecular weight, and solubility. XRD experiments showed that both polysaccharides were adsorbed only on the external surfaces of the clay minerals. Dextran and scleroglucan changed the degree of parallel stacking in Ca-montmorillonite quasicrystals, but the particle arrangement of the starting clays observed by SEM was roughly preserved. The water-stable aggregation, however, increased with scleroglucan. These results on clay-polysaccharide complexes demonstrate the important role played by polysaccharide structure (e.g., tertiary and quaternary structure) in adsorption and clay aggregation.