Supported olefin polymerization catalyst compositions prepared by contacting, previous to or immediately before the beginning of the olefin polymerization, a support comprising a porous carrier, optionally with a) an organometallic compound of the general formula (1): R1MXv-1, wherein each R is the same or different and is a C1-C10 alkyl group; M is a metal of Group 1, 2, 12 or 13 of the Periodic Table; each X is the same or different and one of a halogen atom, a hydrogen atom, a hydroxyl radical or a C1-C8 hydrocarbyloxy group; 1 is 1, 2 or 3; v is the oxidation number of the metal M, and with a complex solution of at least b) a metallocene of the general formula (2): (CpY)mM'X'nZo, wherein each CpY is the same or different and is one of a mono- or polysubstituted, fused or non-fused, homo- or heterocyclic cyclopentadienyl, indenyl, tetrahydroindenyl, fluorenyl, or octahydrofluorenyl ligand, which ligand is substituted at its cyclopentadienyl ring with at least one substituent Y which is one of a -OR', -SR', -NR'2, -C(H or R')=, or -PR'2 radical, each R' being the same or different and being a tri-C1-C8 hydrocarbyl silyl group or a tri-C1-C8 hydrocarbyloxy silyl group; M' is a transition metal of Group 4 of the Periodic Table and bound to the ligand CpY at least in an θ5 bonding mode; each X' is the same or different and is one of a hydrogen atom, a halogen atom, a C1-C8 hydrocarbyl group, a C1-C8 hydrocarbylheteroatom group or a tri-C1-C8 hydrocarbylsilyl group or two X' form a ring with each other; Z is a bridge atom or group between two CpY ligands or one CpY ligand and the transition metal M'; m is 1 or 2; o is 0 or 1; and n is 4-m if Z is a bridge between two CpY ligands or n is 4-m-o if Z is a bridge between one CpY ligand and the transition metal M', and c) an alumoxane of general formula (3), wherein each R'' and each R''' is the same or different and is a C2-C10 alkyl group; and p is an integer between 1 and 40, and recovering said supported olefin polymerization catalyst composition.