The present investigation focuses on the flow-field upstream of co-flowing methane-air partially premixed flames stabilized in a splitter plate burner, with wide variation of the reactant concentration gradient. The velocity field is deduced from particle image velocimetry and the flame structure is visualized by means of planar laser induced florescence of the OH radical. The flame structure changes from that of a nearly non-premixed to a nearly premixed flame as the concentration gradient is varied. The flame curvature increases linearly with the concentration gradient over the wide range tested. The strain component of the flame stretch at the flame leading edge nearly equals the curvature component at a critical concentration gradient; further increase in the concentration gradient beyond this critical value shows that the curvature component progressively dominates over the strain component. The propagation velocity (global flame speed) of the flame increases with reduction in the total stretch until a critical value, below which it decreases even though the stretch reduces further. The local flame speed peaks at the same value of flame stretch as the global flame speed, which is at the critical concentration gradient. This is explained as the balance between the availability and sufficient temperature of excess reactants from the premixed branches, which lead to heat contribution from the non-premixed branch to the leading edge. For flames close to the non-premixed extreme, the strain component of stretch at the leading edge measures less than that along the branches due to the sharp flow divergence upstream. For flames towards the premixed extreme, the flow diverges gradually; for such flames the strain component measures maximum at the leading edge. The changeover occurs at the critical concentration gradient corresponding to peak propagation velocity. © 2013 The Combustion Institute.