The present invention relates to a process for the preparation of a C20 to C60 wax from the thermal decomposition of a particular plastic polyolefin polymer blend. The present invention provides a vacuum pyrolysis process for preparing a C20 to C60 wax from the thermal decomposition of plastic polyolefin polymer, the method comprising the steps of: i) introducing plastic polyolefin polymer into a thermal reaction zone of a vacuum pyrolysis reactor; ii) heating the plastic polyolefin polymer at sub-atmospheric pressure, wherein the temperature in the thermal reaction zone of the reactor is from 500° C. to 750° C., to induce thermal decomposition of the plastic polyolefin polymer and to form a thermal decomposition product effluent which comprises a major portion by weight of a C20 to C60 wax fraction; and iii) condensing a vapour component of the thermal decomposition product effluent from the vacuum pyrolysis reactor; wherein the plastic polyolefin polymer comprises polyethylene and polypropylene in a polyethylene to polypropylene weight ratio of from 60:40 to 90:10.