The steam gasification of different mixtures of biomass and high density polyethylene (0 wt%, 25%, 50% and 100% of HDPE) has been carried out in continuous mode in a conical spouted bed gasifier, and the results have been compared with those obtained feeding pure biomass and HDPE. The experiments have been carried out at 900 degrees C and using a steam/(biomass + plastic) mass ratio of 1. Olivine has been used as primary catalyst in order to minimize tar formation. The cofeeding of HDPE has given way to an interesting synergetic effect involving the reduction of both tar and char yields. Thus, the tar content in the gaseous product was reduced from 58.23 g Nm(-3) in the gasification of pure biomass to only 9.74 g Nm(-3) when a mixture made up of 50% of each feedstock was in the feed. These promising results evidence the interest of the cogasification strategy apart from contributing to increasing feedstock availability. (C) 2015 Elsevier Ltd. All rights reserved.