The operating regime of a gas turbine combustor is highly sensitive to fuel composition changes, In particular, the addition of hydrogen, a major constituent of syngas, has a major effect on flame behavior due to the higher burning rates associated with hydrogen. A laboratory scale premixed test rig is constructed in order to study such effects. The fuel composition is incremented with increasing hydrogen starting from 100% methane. It is observed that increased RMS pressure levels and higher susceptibility to flashback occur with increasing hydrogen volume fraction. Furthermore, hydrogen enrichment can cause an abrupt change in the dominant acoustic mode. Measurements are reported of real-time heat release, emissions and flashback. Particular emphasis is put on understanding the relationship between the thermo-acoustic induced pressure oscillations and flashback. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.