The stainless steel wools often used in high gradient magnetic separation have fibre cross sections which are usually approximately rectangular This paper describes the study of the upstream and downstream magnetic capture on a single wire of this type by using a high-speed video camera and an image processing system. The results obtained are compared with those obtained with a round nickel wire. The different shapes of the magnetic deposit on the wire were observed under different conditions. The mechanism of vortex magnetic separation in the wake area behind the wire was also investigated through the analysis of the particle trajectories. The measurements with the image processing system revealed, at moderate Reynolds number the velocity of the returning flow near the stagnation point is usually smaller than 1 cm/s. This benefits the magnetic capture of the magnetic particles on the downstream side of the wire because the competing force (drag force) caused by the returning flow is much weaker than the drag force caused by the flow around the front of the wire.