The stripper section of a fluid-coker consists of a system of baffles (sheds) that enhances the removal efficiency of entrained and adsorbed hydrocarbons from the fluidised coke-particles. If the particles contain a thin liquid film layer of heavy hydrocarbons, making them excessively wet' or sticky', and if they stay in contact with sheds for too long, solid deposits are formed that lead to stripper fouling. Extensive fouling decreases stripping efficiency and liquid product yield and can shorten run-times between shutdowns. Because of the fouling, the shape of sheds mostly changes by increasing their surfaces thickness. An early indication of that fouling and the ability to follow its development are essential for choosing optimal parameters of the process. The radioactive particle tracking (RPT) method has been tested to determine its applicability to indicate the change in the shape of internals within a fluidised bed reactor when direct observation is impossible. A single radioactive tracer-particle has been traced in experiments lasting from 2 to 6h. The experiments were conducted in a lab-scale, cold-flow fluidised bed into which a single shed with walls of different thickness was incorporated. This experimental fluidised bed provides intensive solid phase mixing that allows a single tracer-particle to be located in any place within the reactor. By registering the frequency of the tracer-particle appearance within a defined internal space surrounding the shed, the shape of shed was reconstructed. The conducted experiments suggest that RPT technique allows for tracking internals' fouling within a fluidised bed reactor. (c) 2012 Canadian Society for Chemical Engineering