Process for the preparation of a solid olefin polymerization catalyst system, comprising an organometallic compound of a transition metal of Group 3 to 10 of the Periodic Table (IUPAC 2007) in the form of solid particles comprising the steps of I) generating an emulsion by dispersing a liquid clathrate in a solvent (S) wherein (i) the solvent (S) constitutes the continuous phase of the emulsion and comprises a nonreactive fluorinated synthetic oil having a viscosity at 20° C. according to ASTM D445 of at least 10 cSt up to 2000 cSt (ii) the liquid clathrate constitutes in form of droplets the dispersed phase of the emulsion, II) solidifying said dispersed phase to convert said droplets to solid particles and III) optionally recovering said particles to obtain said catalyst system, wherein the liquid clathrate comprises (a) a lattice being the reaction product of a1) a transition metal compound of formula (I) LmRnTXq wherein “T” is a transition metal of anyone of the groups 3 to 10 of the periodic table (IUPAC2007), preferably a transition metal of anyone of the groups 4 to 6 of the periodic table (IUPAC2007), more preferably titanium (Ti), zirconium (Zr) or hafnium (Hf), i.e. zirconium (Zr) or hafnium (Hf), each “X” is independently a monovalent s-ligand, each “L” is independently an organic ligand which coordinates to the transition metal (T), “R” is a bridging group linking said organic ligands (L), “m” is 2 or 3, preferably 2, “n” is 0, 1 or 2, preferably 1, “q” is 1, 2 or 3, preferably 2, m+q is equal to the valency of the transition metal (T), a2) a cocatalyst comprising aluminoxane a3) a compound being effective to form the lattice with the transition metal compound and/or the aluminoxane and b) a hydrocarbon solvent (HS).