High pitch rate manoeuvres have been studied in great depth in recent decades, in respect to rotor dynamic stall and, more recently, in understanding bird and insect flight with view to application in biomimetic micro air vehicles (MAVs). The flow topology arising from such unsteady airfoil movement is complex. This paper focuses on assessing the development of spanwise variation and three-dimensional substructure within the leading edge vortex (LEV) produced by a rapidly pitching airfoil. Two-component, two-dimensional (2C-2D) particle image velocimetry (PIV) is used to take measurements of the flow in a plane above the upper surface of an airfoil at two different Reynolds numbers. Observations conclude that flow three-dimensionality develops in the form of coherent vortical structures along the dividing stream surface (DSS) upstream of the leading edge vortex. The average spanwise spectral content of the flow immediately upstream of the leading edge vortex is calculated, allowing quantitative analysis of the dependence of leading edge vortex substructure on Reynolds number and time.