Fuel flexibility in stationary gas turbines (GT) is becoming increasingly important due to the use of a broader spectrum of primary energy sources, particularly H₂-rich fuels derived from the gasification of coal or biomass. GTs also must be able to operate at extremely low emission levels, which is currently achieved with lean-premixed burner designs. To investigate the performance of highly reactive fuels in the reheat combustion concept, mainly with respect to autoignition and flashback limits, a generic reheat combustor with excellent optical access has been developed. The first objective of this work was to carefully characterize the mixing section in order to derive well-defined boundary conditions for the subsequent autoignition studies. Initial autoignition results at T > 1000 K and p = 15 bar are presented for natural gas (NG) and H₂-rich fuels. No autoignition was detected for NG at the investigated operating conditions. For H₂/NG/N₂ blends with a constant volumetric N₂ concentration of 20% and H₂ concentrations higher than 76%, autoignition in the mixing section was detected.