Magnox Electric maintains a significant microstructural programme in support of its safety case for operation of its stations with steel pressure vessels. An important part of this programme is the characterisation of grain boundary chemistry using Auger spectroscopy. Mechanical testing and subsequent examination of surveillance material has shown that some Charpy specimens display a proportion of intergranular fracture and Auger work has linked this to the presence of phosphorus on the grain boundaries. A feature of particular interest in the study of the boundaries is the co-segregation of carbon. The measurement of the ‘true’ levels of phosphorus and carbon segregation is complicated by the presence of carbon contamination. This paper describes the simple approach used to overcome this problem.