Chlorinated olefin polymers prepared from olefin polymer base resins having I10 values of 0.05-0.8 dg/minute act as impact modifiers for vinyl chloride polymers under low to ambient temperature conditions. When vinyl chloride polymers are mixed with a preferred class of the chlorinated polymers, having chlorine contents of 20-30 percent by weight, compositions that exhibit short fusion times and low fusion temperatures are produced.