The utilization of pre-treated polyolefin rewastes with high-boiling hydrocarbon fractions was investigated as alternative of raw material recycling. Pyrolysis of selected mixtures into lower olefins and olefinic intermediates has been performed in a flow reactor in the presence of steam (or inert gas) as diluent. The effect of feed composition, residence time (0.3-1.23), and temperatures (580--820 degrees C) on product yields was studied. The addition of pre-treated plastics in specific percentages (up to 15 wt-%) does not lead to any significant changes in the yields. Coke formation on metal surfaces during the thermal conversion of the various hydrocarbon mixtures was additionally studied using a gravimetric micro-balance. An effect of feed composition on the coke amount could not been observed.