The change in harmonic content of oscillations as they propagate through a plant is modelled by examining ratios of observed signals. Heuristics are then proposed to enable these models to be characterised without recourse to detailed plant identification. This results in a harmonic-content based procedure to isolate the source of an oscillation. The procedure is tested on bi-amplitude ratio data generated from measurements obtained from a real industrial plant. (C) 2006 Elsevier Ltd. All rights reserved.