Magnetic Barkhausen emissions and magnetic hysteresis measurements were carried out on water-quenched 5Cr-0.5Mo steel subjected to ageing at 600 °C up to 5000 h. During initial ageing, this steel exhibited magnetic softening, which was attributed to relaxation of quenching stress in the material as well as decrease in dislocation density and migration of interstitial carbon atoms towards the grain boundary. Further ageing resulted in magnetic hardening owing to the restricted movement of the domain wall by the precipitation of carbides such as M3C2, M2C, M7C3 where M stands for Fe, Cr or a combination of them. At longer ageing periods, magnetic behaviour was affected by a change in the composition and morphology of the carbides. Massive M23C6 types of carbides were formed during longer periods of ageing. The coarsening of carbides decreased the pinning density for the domain wall motion and affected the magnetic properties of the steel. The effect of demagnetizing field from voids and non-magnetic massive carbides also affected the magnetic behaviour. Magnetic behaviour and Vickers hardness measurements during ageing have been effectively supported by microstructural evaluations suggesting the capability of the magnetic techniques for assessment of damage during ageing in high temperature 5Cr-0.5Mo steel components. © 2009 IOP Publishing Ltd.