The present invention relates to a process to prepare alkylaluminoxanes by reaction of alkylaluminium with methacrylic acid or a conjugated unsaturated carbonyl-functional compoundof the formula (I) wherein each R1and R2 independently are an aliphatic hydrocarbon group, and R3 independently is the same hydrocarbon group as R1 and R2 or a hydrogen atom, and R4 isanaliphatic hydrocarbon group, a hydroxyl group or a hydrogen atomin the presence of an inert organic solvent. Additionally,it relates to the alkylaluminoxanes obtainable by the above process and their use.