This study aims at comparing the steam gasification behaviour of two species of algal biomass (Chlamydomonas reinhardtii and Arthrospira platensis) and three species of lignocellulosic biomass (miscanthus, beech and wheat straw). Isothermal experiments were carried out in a thermobalance under chemical regime. Samples had very different contents in inorganic elements, which resulted in different reactivities, with about a factor of 5 between samples. For biomasses with ratio between potassium content and phosphorus and silicon content K/(Si + P) higher than one, the reaction rate was constant during most of the reaction and then slightly increased at high conversion. On the contrary, for biomasses with ratio K/(Si + P) lower than one, the reaction rate decreased along conversion. A simple kinetic model was proposed to predict these behaviours. (C) 2014 Elsevier Ltd. All rights reserved.