The advantages of supporting polymerization catalysts on oxides include better polymer morphology, decreased cocatalyst requirements and suppression of reactor fouling. However, the interaction between the catalyst and the oxide support can also change the nature of the active site, resulting in lower activity, altered polymer microstructure and increased polydispersity. While heterogeneity of the active sites usually presents a barrier to understanding the origin of these effects, the ability to synthesize uniform active sites with a combined molecular-surface approach can enable structural and mechanistic studies and, ultimately, the reengineering of more efficient catalysts. A conventional Phillips catalyst and a single-site catalyst, both activated by oxides, will be described using a combination of catalyst characterization and polymer characterization techniques.