The interaction of a premixed methane-air flame with a two-dimensional counter-rotating vortex pair is studied under stoichiometric and rich conditions using a detailed C1C2chemical mechanism. The flame structure and transient response are examined, both at curved cusps and on the vortex-pair centreline. Differences between the two flames are observed in the unsteady behaviour of species mole fractions and production rates. In contrast with earlier one-dimensional opposed-jet flame data, the present results show that the rich flame exhibits a faster response to unsteady strain-rate disturbances than does the stoichiometric flame. Analysis of the results suggest this may be due to the increased dependence of the flame on H, and the decreased role of OH, under rich conditions. Results are also presented from an experimental V-flame vortex-pair interaction study. Measured peak CH and OH data are also found to exhibit a faster flame response under rich conditions.