Catalysts for the polymerization of olefins which comprise the reaction product of (a) a bridged and/or substituted cyclopentadienyl compound of titanium, zirconium, or hafnium; (b) an organometallic aluminum compound of formula Al(CH.sub.2 --CR.sup.4 R.sup.5 R.sup.6).sub.w R.sup.7.sub.y H.sub.z, wherein in the (CH.sub.2 --CR.sup.4 R.sup.5 R.sup.6) groups, which are the same or different, R.sup.4 is an alkyl, alkenyl or arylalkyl group having from 1 to 10 carbon atoms, R.sup.5 is an alkyl, alkenyl, aryl, arylalkyl or alkylaryl group having from 3 to 50 carbon atoms and, optionally, R.sup.4 and R.sup.5 can be fused together to form a ring having from 4-6 carbon atoms, R.sup.6 is hydrogen or an alkyl alkenyl or arylalkyl group having from 1 to 10 carbon atoms, the R.sup.7 substituents, the same or different, are alkyl, alkenyl, aryl, arylalkyl or alkylaryl radicals having from 1 to 10 carbon atoms and, optionally, can contain Si or Ge atoms, w is 1, 2 or 3, z is 0 or 1, and y=3-w-z; and (c) water. The molar ratio of aluminum to water is from 1:1 to 100:1. The catalysts show an improved activity with respect to known catalysts wherein an aluminum compound different from those of the invention are used.