The inhibition of water-soluble cationic graft terpolymers of hydroxyethylcellulose with the vinyl monomers dimethyldiallylammonium chloride and acrylamide to the hydration of water-sensitive clay in oilfields was investigated by clay hydration-swelling tests, XRD analyses, zeta-potential measurements, and SEM observation. It has been shown that grafting enhances the hydration-inhibition effectiveness and the performances of these cationic cellulosic polymers are strongly dependent on their cationicity, relative molecular weight, and solution concentration.