Nevertheless, the yield of hydrogen that can be produced from biomass is relatively low, at about 5e8 wt.% based on dry biomass [15]. In order to increase the amount of hydrogen in the gaseous stream, the addition of plastics to biomass wastes is an interesting alternative, in that they contribute to increasing the content of hydrogen in the feed. Furthermore, since the availability of biomass wastes is subject to seasonal changes, the addition of another material would help to maintain a steady supply of the feedstock during off-season of a given biomass. Polypropylene (PP), polyethylene (PE) and polystyrene (PS) are the three main plastics present in municipal solid wastes. Adding PE, PP or PS to biomass wastes can lead to an increase in the gas and hydrogen yield since polyolefinic plastics are an excellent hydrogen source. Furthermore, these materials can also be very useful for the selective production of higher value added products, such as light olefins [16].