The soluble catalyst system methylaluminoxane (MAO)-Ni(acac)(2) (acac: acetylacetonate) gives polystyrene consisting of an amorphous (aPS) and a crystalline isotactic (iPS) fraction. Al(CH3)(3), which is always present in commercial samples of MAO, decreases both polymer yield and stereospecificity. The polymer yield increases with increasing the MAO/Ni ratio but, at the same time, the iPS/aPS ratio decreases. Addition of N(C2H5)(3) (mole ratio N/Ni = 1) increases the proportion of the isotactic fraction, while it decreases the polymer yield. A tentative interpretation of the stereospecificity is reported.