AbstractLarge amounts of ash, generated from biomass gasification, often contaminate syngas and the ecosystem. This study showed that the ash obtained from the gasification of pine wood was primarily composed of carbon (15% to 25%), minerals (~21%), and oxygen (52% to 63%), and exhibited low surface area (8.4 to 11.2 m2/g). The size of ash particles was between 600 nm and 600 μm. Calcium, potassium, and sodium were the three most common mineral elements in the ash. Leaching tests showed that adding ash to water raised the pH value from 5.7 to between 11.2 and 11.5, and, as time progressed, more mineral elements were released from the ash. For growing microalga Chlamydomonas debaryana in media containing ashes, no toxicity of pine ash was found.