A 1.5 kWth hybrid solar/autothermal steam gasifier was designed and tested for continuously producing synthesis gas. Time-averaged responses to a series of inlet flow conditions were examined in a high-flux solar simulator. The gasifier consisted of a SiC absorbing/emitting tube inside of a windowless Al2O3-SiC cavity containing a fluidized bed indirectly irradiated by concentrated irradiation. Monte Carlo ray-tracing was performed to examine heat flux distributions and determine optimal prototype geometry and lamp settings. An experimental design with replicates was used to examine the effects of H2O:C, O2:C, and feedstock with two-way analyses of variance. Carbon conversions of up to 0.79 were achieved, and maximum cold gas ratio and solar-to-fuel efficiency were 1.16 and 22%, respectively. Introduction of O2 led to a significant increase in bed temperature. Higher performance was observed using activated charcoal compared with lignite coal. The gasifier results indicate that a hybrid solar/autothermal gasification process may be used to overcome disturbances due to solar transients.