Styrene polymerization was carried out with Ni(acac)(2)/MAO and Ni(acac)(2)/SiO2/MAO. The influence of reaction parameters (Al/Ni mole ratio, catalyst concentration, temperature and time polymerization) on styrene polymerization was evaluated. It was observed that both catalytic systems were affected by reaction parameters and that the heterogeneous catalyst presented higher activity than the homogeneous one. Polystyrenes with different molecular weight, stereoregularity and polydispersity were obtained. These results suggest that different active catalyst species could have been present. In addition, two types of methylaluminoxane (MAO) with different moleculax weights were also evaluated as cocatalyst. As a result. the catalyst activity and stereospecificity were strongly affected by the MAO type,