The kinetics of the polymerisation of propene have been studied in methylene chloride solution at –15, –35, and –50°, with use of boron fluoride–methanol mixtures as catalyst. The rate expression for the disappearance of monomer was found to be –dM/dt=k[BF₃]_soln[CH₃OH·BF₃]^3/2[M] and k was virtually independent of temperature. The molecular weight of the product increased from about 300 to about 1000 as the temperature was reduced from –15 to –80° and was independent of monomer concentration but increased slightly with co-catalyst concentration.