This paper presents a study of the effects of different treatments on the polymerization activity of modified clays as cocatalysts. To achieve this goal, an intercalating cation was introduced into two smectites and these clays were then modified with trimethyl aluminium. The results for ethylene polymerization, when a zirconocene complex was used as catalyst, and the structure analysis, allow us to obtain interesting deductions about the generation mode of the active species. All active materials employed as support activators presented aluminium in a pentahedral environment together with acidic hydrogen atoms. These two features were detected only after TMA treatment and they seem to be crucial elements in active cocatalyst generation. Moreover, a material without structural aluminium displayed the best activity pointing to the new aluminium species generated in the solid matrix as the determining factor for the activity. We proposed a synergic effect between Lewis acid aluminium centres and acidic Bronsted protons that generate the SiOHAl groups that activate the zirconium compound. Published by Elsevier B.V.