The effects of unequal pitching and plunging frequency on the flow structures around a flapping airfoil is investigated using the Digital Particle Image Velocimetry (DPIV) technique in the reduced frequency range of 0.31 4 <= k <= 6.26 corresponding to Strouhal number range of 0.1 <= St <= 1.0. The SD7003 airfoil model undergoes a combined flapping motion where the pitch leads the plunge with a phase angle of 90 in a steady current. The investigated cases are classified into five flow structure categories based on instantaneous and averaged vorticity patterns and velocity fields around and in the near-wake of the airfoil while the frequency of plunging motion was kept the same as the frequency of pitching motion. Example cases for each category were then investigated for unequal pitching and plunging frequencies and it is observed that employing unequal pitching and plunging frequencies for an oscillating airfoil may result in a change of flow structure category. (c) 2014 Elsevier Masson SAS. All rights reserved.