Supported catalyst system for the polymerization of olefins, having at least two different monocyclopentadienyl transition metal compounds, one or more activators including an ionic compound having (i) a cation and (ii) an anion having up to 100 non-hydrogen atoms and the anion containing at least one substituent comprising a moiety having an active hydrogen, and one or more support materials. The supported “mixed or dual site” catalyst systems having different monocyclopentadienyl catalysts when activated by specific ionic activators lead to catalyst systems showing an improved balance of properties which may be used to prepare LLDPE polymers having broad melt flow ratios.