The presence of hydrogen and/or impurity segregation can significantly affect grain boundary strength of iron. Few studies, however, have been attempted to correlate grain boundary strength to impurity concentration along grain boundaries. In this study, grain boundary strength will be quantitatively correlated to impurity segregation in iron with or without the presence of hydrogen. The impurity concentration along grain boundaries was controlled by using iron specimens of different initial bulk impurity concentrations, doping and heat treatments. Quantitative analysis of segregated impurities was carried out using Auger electron spectroscopy with in-situ fracture under ultra high vacuum. The segregation of sulphur clearly decreased the grain boundary strength in both hydrogen-precharged and uncharged iron specimens. On the other hand, the presence of a small amount of carbon can increase grain boundary strength substantially. In the light of present observations, the mechanism of hydrogen-induced inter-granular fracture and the effects of sulphur, carbon and other impurities will be examined.