This article presents an investigation of the swirl number effects on the stability limits of various syngas compositions in an atmospheric premixed variable-swirl burner. Lean blowout (LBO) and flashback (FB) experiments were performed using fuel mixtures containing varying amounts of H2, CO and CH4 at two swirl numbers. Reducing the swirl number from 0.66 to 0.53, reduced the flashback propensity of various syngas/air mixtures but it did not affect the LBO limits considerably. The flow-field in the combustor was studied at S = 0.66 and 0.53 using high-speed particle image velocimetry (PIV) for a mixture of H2/CH4 (50:50) at various equivalence ratios. At both swirl numbers, for the non-reacting flow and low equivalence ratios the flow-field consisted of an inner recirculation zone at the entrance of the combustor, an annular high velocity zone and an outer recirculation zones between the high-velocity zone and the bounding walls. Increasing the equivalence ratio towards the flashback limit, had varying effects on the flow-field, depending on swirl number. At S = 0.66, increasing the equivalence ratio did not have a significant effect on the general features of the flow-field. At S = 0.53 the flow-field consisted of an inner recirculation zone at low equivalence ratios, but as the equivalence ratio was increased, the high velocity zone extended radially towards the center and the recirculation zone disappeared from the flow-field. The high-speed OH*-chemiluminescence images recorded at the onset of flashback revealed significant differences between S = 0.66 and S = 0.53 in the flame stabilization mechanism prior to flashback and flame propagation in the premixing tube. Considering the velocity measurements together with the OH*-chemiluminescence images, it was concluded that at S = 0.66 flashback was caused by CIVB mechanism whereas at S = 0.53 flashback was initiated by the competition between the flame speed and flow velocity in the core flow. Copyright © 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.