Decarburisation of high carbon steel has been simulated, using composite samples comprised of a 316 stainless steel (paramagnetic) core and a surrounding tube of ferritic steel (ferromagnetic) with thicknesses between 100 and 600 μm, for determining the potential for on-line measurement during steel processing. Decarburization samples have also been generated, for off-line measurements, by heat treatment of an Fe-0.8 wt% C steel for various times in air at 1000–1200°C. A multi-frequency (10–10⁶ Hz) electromagnetic sensor was used to determine variations in inductance (due to differences in permeability) as a function of decarburisation depth. The relationship between sensor output and decarburised layer type/thickness has been modelled using finite element software.