Vortex shedding behind plates with rectangular trailing edges and both aerofoil and rectangular leading edges was investigated in a water tunnel. Previous studies, such as that presented by Mills et al. (1995), have shown that the base pressure, and hence drag, of such plates can be significantly altered by applying transverse perturbations at certain frequencies. Evidence has been presented to show that this is dependent on the interaction of shed vortices; this study is a first attempt to study the nature of the vortices and how they are influenced by the applied perturbation. This is done by using particle image velocimetry to measure the velocity field close to the plates. From these the vorticity field is derived and its response to perturbation examined. It is shown that the perturbation orders the vorticity field, resulting in increased peak vorticity and circulation in the shed vortices. Thus, their potential for inducing greater suction in the wake has been increased.