Catalysts useful in olefin polymerization are a mixture of (a) (i) one or more compounds Zn(X.sup.1).sub.2 and one or more compounds Al(R.sup.1).sub.3, or (ii) Zn(X.sup.1).sub.2.2Al(R.sup.1).sub.3, wherein X.sup.1 is halide and R.sup.1 is C.sub.1-12 hydrocarbyl; and (b) one or more of V(X.sup.2).sub.c (OR.sup.2).sub.b-c, VO(X.sup.3).sub.d (OR.sup.3).sub.3-d, or VO(X.sup.4).sub.2, wherein X.sup.2, X.sup.3 and X.sup.4 are halogen, R.sup.2 and R.sup.3 are C.sub.1-18 hydrocarbyl, b is 3-4, c is 0-b, and d is 0-3; and can contain (c) one or more of M(R.sup.5).sub.e (X.sup.5).sub.3-e, Al.sub.2 (R.sup.5).sub.3 (X.sup.5).sub.3 or Mg(R.sup.6).sub.f Y.sub.2-f, wherein X.sup.1 and X.sup.2 are halide, R.sup.1, R.sup.5, R.sup.6 and R.sup.9 are C.sub.1- 12 hydrocarbyl, M is Al or B, e is 0-3, Y is halide, O--(C.sub.1-12 hydrocarbyl) or N(SiR.sup.9.sub.3).sub.2, and f is 0-2.