NOTICE: this is the author’s version of a work that was accepted for publication in the International Journal of Hydrogen Energy. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published at: http://dx.doi.org/10.1016/j.ijhydene.2012.08.061Time-Resolved Particle Image Velocimetry was used to study transient interactions between hydrogen-enriched methane/air premixed flames and toroidal vortex structures. Lean and stoichiometric mixtures with hydrogen mole fraction in the fuel (hydrogen plus methane), xH2xH2, varying in the range of 0–0.5 were investigated.\ud \ud Results have shown that the hydrogen presence affects the flow field both quantitatively (increase of the velocity of the main toroidal vortex) and qualitatively (generation of different sub-vortices within the main vortex), enhancing the intensity of the interaction. Regardless of the mixture stoichiometry, the hydrogen substitution to methane leads to a transition from a regime in which the vortex only wrinkles the flame front (xH20.2xH2>0.2). This transition was characterised in terms of time histories of flame surface area and burning rate