The working hypothesis for the study was that the main part of the chlorine in biomass is in an inorganic form and therefore should not vaporize appreciably below the melting point of the corresponding salt (around 700 degrees C) because the vapor pressure over solid salt is negligible. In the study, biomass fuels (sugarcane trash, switch grass, lucerne, straw rape) were subjected to pyrolysis in a flow of nitrogen, and the weight of the residue and its chlorine content were measured and compared to the original fuel. Contrary to the hypothesis, the results showed that during pyrolysis of biomass 20-50% of the total chlorine evaporated already at 400 degrees C, although the majority of the chlorine was water soluble (in grass 93%) and therefore most probably ionic species. At 900 degrees C, 30-60% of the chlorine was still left in the char. At 200 degrees C less than 10% of chlorine had evaporated from the fuel, indicating that the chlorine is rapt associated with water. Another result was that there was no significant difference in the chlorine release between biomass and synthetic waste, i.e., a mixture of organic and inorganic chlorides. These results are contradictory with the starting hypothesis and can therefore have new implications for the use of these fuels in combustion and gasification processes.