In this work, experiments concerning supercritical water gasification have been performed. High pressure tests at 300 bar were conducted, operating at temperatures of 350°C and 400°C with different reactor materials namely stainless steel and Inconel® 625. Micro¬autoclaves were used for testing, which allowed great ease of use. Product gas composition, as well as total organic carbon in the produced liquid, were measured at different reaction times. Tests with glucose revealed that, at supercritical conditions, a stainless steel reactor is able to produce a gas with higher hydrogen content. Long¬time (16 h) tests were also performed with some types of biomass, with and without addition of K2CO3 as catalyst. The results confirm the higher hydrogen selectivity of stainless steel only for some feedstock, while the beneficial effect of the catalyst is always evident.