A magnesium titanium olefin polymerization procatalyst is prepared by A) reacting a diorganomagnesium compound with a source of active chlorine, (with the proviso that the amount of chlorine is insufficient to completely convert the diorganomagnesium to magnesium dichloride); then B) removing unreacted diorganomagnesium from the reaction product; then depositing a tetravalent titanium species on the reaction product. This procatalyst is highly active for the solution polymerization of olefins when combined with a cocatalyst.