Quantitative evaluation of time dependent flow structures around and in the near-wake of an oscillating airfoil is investigated using the Digital Particle Image Velocimetry (DPIV) technique to perform a detailed categorization of vortex formations in the reduced frequency range of 0.16 <= k <= 6.26 corresponding to Strouhal number range of 0.05 <= St <= 1.0. The SD7003 airfoil model known to be optimized for low Reynolds number flows undergoes a combined motion where the pitch leads the plunge motion by psi = pi/2 in a steady current. Five flow structure categories are identified depending on the role of separated vortex structures from the leading and trailing edges. The occurrence of flow structure categories on different two-dimensional parameter spaces is obtained. It is also found that the categorizations are independent of the Reynolds number for the investigated range. (c) 2012 Elsevier Ltd. All rights reserved.