Olefin polymerization catalysts capable of polymerizing olefins such as ethylene to polyolefins with broad molecular weight distributions are made by depositing on an oxide suport (or especially phosphate derivative thereof) (i) a chromium complex of formula RCr-L, where R is optionally substituted pentadienyl or cyclopentadienyl and L is a reactive ligand, or a bis arene chromium and (ii) a metal complex of formula MR³ R⁴ R⁵ R⁶ wherein M is Ti, Zr or Hf, and R³-R⁶ can be allyl (when M is Zr or Hf) or of 4-20 carbons of formula (a) CH₂ X (R⁷ R⁸ R⁹) where X = C, Si or Ge and R⁷-R⁹ are hydrocarbyl, or (b) CH₂ Ar where Ar is optionally substituted aromatic, or said metal complex is of formula Ti R¹¹ R¹², where R¹¹ and R¹² are acyclic dienyl.