A catalyst system solution obtainable by a process comprising the following steps: (a) contacting a solution of methylalumoxane in an aromatic solvent (solvent a) with a solution of one or more organo-aluminium in a solvent (solvent b), or a solution of one or more alumoxanes different from methylalumoxane in a solvent (solvent b); (b) when solvent b) is an aromatic solvent or if solvent b) has a boiling point lower than solvent a) add to the solution formed in step a) an alifatic solvent (solvent c) having a boiling point higher than solvent a) and solvent b); or (c) solubilizing a metallocene compound in the solution obtained in step a) or in step b); and (d) substantially removing solvent a) or solvent a) and solvent b) from the solution.