AbstractGrain boundary concentrations of phosphorus were determined using Auger electron spectroscopy in a 12Cr-Mo–V (wt-%) steel doped with 0·057%P after annealing at 450, 550, and 600°C for times up to 10 000 h. Data were calculated for the thermodynamics of the segregation equilibrium and diffusivities in the segregation kinetics and compared with the corresponding data for Fe–P alloys and with bulk and grain boundary diffusivities of phosphorus in the steel determined using a radioactive tracer method. It is concluded that segregation and diffusion in 12Cr–Mo–V steel is strongly affected by a high dislocation density, the dislocations are stabilised by fine carbide precipitates even after aging for long times at high temperatures and act as traps for phosphorus.MST/1525