A correlation between grain boundary etching of a martensitic steel and phosphorus segregation was established. Using metallographic experiments, the phosphorus diffusivity D in the steel and the Gibbs free energy ΔG P of phosphorus grain boundary segregation were determined: D=0.183 - 0 . 1 7 4 + 3 . 4 7 5 x10 - 4 exp(-229+/-19kJmol - 1 /RT) m 2 s - 1 , ΔG P =-43.1 kJmol - 1 .