The segregation of phosphorus to grain boundaries is well known to cause embrittlement in a range of ferritic pressure vessel steels. Modelling of segregation is therefore useful in predicting the service life of pressure vessels, particularly in those pressure vessels used in nuclear reactor power generating plant where significant cost savings can be made. In this paper, predictions of the radiation induced grain boundary segregation of phosphorus in ASME SA533B pressure vessel steels are made using an analytical model for impurity segregation in dilute ternary alloys. The model takes into account equilibrium-type segregation, together with a newly adopted approach incorporating the site competition effects that exist between phosphorus and carbon. The site competition effect causes the predicted phosphorus segregation to fall to levels significantly lower than the predictions obtained from models of dilute binary alloys. The predictions are compared to FEGSTEM results of samples irradiated at between 255°C and 315°C, doses between 1.5 and 38mdpa and dose rates ranging from 6×10-9 to 8.9×10-11 dpa/s. The predictions obtained from the model are in close agreement to the experimentally observed segregated phosphorus results.