The reactivity of fuel-air mixtures has a strong impact on the severity of obstacle-induced turbulent explosions. While the influence on the flame speed and pressure development has been studied for a wide range of mixtures at different physical scales, the experimental quantification of the resulting flow fields, e.g. in the critical recirculation zones behind obstacles, is typically absent. The lack of such data presents a serious impediment to the development of reliable predictive methods. The current study reports velocity measurements obtained from highly reproducible experiments performed in a flame tube with two staggered obstacles using fuel lean H 2 /CH 4 /Air and H 2 /CO/Air mixtures at a stoichiometry of 0.60. The mixture reactivity for both fuels was varied using H 2 substitution levels of 50% and 80% with the pure hydrogen (100%) case used as a reference. The flow field was quantified using high-speed (10 kHz) particle image velocimetry (PIV), time-series PIV and Mie scattering. The time-resolved evolution of the recirculation zone behind the second obstacle was successfully captured with the explosion over-pressure and flame propagation speed also measured. Data is presented for the mean horizontal (u¯) and vertical (v¯) velocity components at 18 spatial locations for each mixture along with the translational velocities of the shear driven recirculating eddies formed behind the second obstacle. It is shown that the temporal evolution of the flows (not velocity magnitudes) can be approximately normalised based on the flame arrival at the second obstacle. The data provides a comprehensive quantitative description of the flow field evolution leading up to explosion events and is expected to facilitate model development.