Novel catalyst systems which comprise one or more diimine nickel (II) or palladium (II) complexes, one or more cocatalysts, and aluminophosphate are disclosed. Olefin polymerization processes using those catalyst systems are also disclosed. The inclusion of aluminophosphate can improve the activity or productivity of such catalyst systems, making such catalyst systems active or more active in olefin polymerization under conditions in which they had previously been inactive or insufficiently active.