Improved process for the preparation of an unsupported, heterogeneous olefin polymerization catalyst system, comprising an organometallic compound of a transition metal of Group 3 to 10 of the Periodic Table (IUPAC) or of an actinide or lanthanide in the form of solid particles comprising the steps of a) preparing a solution of catalyst components, including an aluminoxane, a compound being effective to form stable, liquid clathrates with aluminoxane and an organometallic compound of a transition metal of Group 3 to 10 of the Periodic Table (IUPAC 2007) or of an actinide or lanthanide, in a hydrocarbon solvent, yielding a two phase system with an upper solvent layer, which is separated, b) preparing a liquid/liquid emulsion system comprising a continuous phase in which said solution of the catalyst components forms a dispersed phase in the form of droplets, c) solidifying said dispersed phase to convert said droplets to solid particles and optionally recovering said particles to obtain said catalyst system, the catalysts system itself and its use for olefin polymerization.