Considering the limited turndown potential of gasification technologies, supplementing a fuel cell turbine hybrid power system with natural gas provides flexibility that could improve economic viability. The dynamic characterization of fuel composition transients is an essential first step in completing the system identification required for controls development. In this work, both open loop and closed loop transient responses of the fuel cell in a solid oxide fuel cell (SOFC) gas turbine (GT) hybrid system to fuel composition changes were experimentally investigated using a cyber-physical fuel cell system. A transition from methane lean syngas to methane rich gases with no turbine speed control was studied. The distributed performance of the fuel cell was analyzed in detail with temporal and spatial resolution across the cell. Dramatic changes in fuel cell system post combustor thermal output or "thermal effluent" resulting from anode composition changes drove turbine transients that caused significant cathode airflow fluctuations, by as much as 8% in less than a minute. In comparing the open loop responses to identical tests conducted under closed loop conditions without significant airflow changes, it was discovered that the cathode airflow change was a major linking event in short-term system transient response. The results suggested that modulating cathode air flow in response to fuel composition changes offers promise for the dynamic control of SOFC/GT hybrid systems with fuel flexibility. (C) 2016 Elsevier Ltd. All rights reserved.