The non-linear response of a swirl stabilised, lean premixed flame (CH4/air) was determined by forcing the flame acoustically at frequencies between 40 and 200 Hz with increasing amplitude. Measuring the chemiluminescent emission from OH* with a photodiode sensor and calculating the flame transfer function, a linear response to increasing amplitude was observed at 40 and 60 Hz for all amplitudes with an equivalence ratio phi = 0.56. However, between 80 and 200 Hz the flame response exhibited non-linear characteristics for r.m.s velocity fluctuations greater than 20% of the mean flow velocity. With phi = 0.48, even 60 Hz became non-linear. Phase-locked Particle Image Velocimetry and Intensified CCD imaging were deployed at three amplitudes for detailed study of the flame and flow field response to forcing. At low frequencies the flow field was characterised by a pulsating inner recirculation zone, whilst at all frequencies the outer recirculation zone was modified by vortices rolling up the annular jet. As the forcing amplitude was increased, the effect on the flame shape became more pronounced, with large variations in flame volume at low frequencies and flame extinction due to stretching of the flame around the roll-up vortices at the higher frequencies. The results indicate different driving mechanisms behind the flame response at low and high frequencies. At low frequencies the flame response is governed by equivalence ratio fluctuations due to the 'stiff' fuel system and the volumetric fluctuations of the input air. At the higher frequencies the response is governed by flow field features such as vortex roll-up. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.