An experimental study of a turbulent backward-facing step (BFS) was undertaken to investigate the vortex structures behind the step. Attention was given to the secondary vortex because of its poor representation in literature and its potential for evaluating computational turbulence models. A 2D, cross-correlation particle image velocimeter (PIV) was developed, which allowed measurement of the highly turbulent, reversing step flow. Global, high resolution data was obtained for the cross-sectional plane of the BFS and for several other planes parallel to it. Measurement planes across the step revealed the 3D nature of the secondary vortex and an unexpected flow structure was identified. The secondary vortex was found to traverse across the flow, from the cross-sectional plane towards the step edge-sidewall corner.