The flow of foods in cans is critical in determining process times and thus product quality. The flow of liquids has been followed in axially rotated metal food cans using positron emission particle tracking. The technique involved placing a small (approximate to600 mum) isokinetic radioactive tracer into a container and following its movement with a positron sensitive camera. The small size of the tracer, as well as the ability of the emitted gamma rays to penetrate both opaque fluids and considerable thicknesses metal (approximate to10 cm) make the method ideal for studying fluid flows inside real equipment. The method enables a spatial resolution of close to one millimetre and a time resolution of a millisecond. Flows have been studied in axially rotated cans filled with Newtonian liquids of differing viscosities and a varying amount of headspace. The inclusion of a headspace in a can caused the flow within the two-phase system of a viscous solution to become inherently three-dimensional with a regular period of circulation and flow trajectory. (C) 2003 Elsevier Ltd. All rights reserved.