The experimentally determined data-enthalpy and entropy-of the grain boundary segregation of substitutional solutes of 14th and 15th groups of the periodic table, i.e., silicon, phosphorus, tin, and antimony, in alpha-iron are compared. The consequences of the grain boundary segregation of these elements for the intergranular strengthening or embrittlement are also shown. It is documented that all these solutes except silicon segregate at the grain boundaries interstitially at enhanced temperatures although substitutional segregation is preferred at zero K. Despite some variations, the values of the strengthening/embrittling Gibbs energy of all solutes are nearly linearly dependent on the differences in the sublimation energies of the host and solute.