The grain boundary segregation in two Fe-Si base alloys with the different bulk phosphorus contents was investigated. The phosphorus grain boundary concentrations determined with the use of Auger electron spectroscopy were related to the microstructure, fracture mode, and mechanical properties of the alloys. Transgranular cleavage was found to be the dominant fracture mode of the samples tested by impact at room temperature. The correlations between the phosphorus grain boundary concentration, the portion of intergranular fracture, and the dynamic fracture toughness were presented, which fulfil the theoretical expectation. Two possibilities were proposed to explain a discrepancy in the temperature dependence of the phosphorus grain boundary concentration for the alloy with the higher bulk phosphorus content.