Process for the preparation of a solid catalyst system comprising the steps of preparing a liquid clathrate comprising a mixing of an aluminoxane, an aromatic compound and an organo-silicon compound, wherein the mol-ratio between the organo-silicon compound and aluminum of the aluminoxane is below 0.1, mixing said liquid clathrate with an organometallic compound obtaining a liquid mixture, and precipitating the solid catalyst system out of said liquid mixture by adding a saturated aliphatic compound to said mixture liquid.