Thermodynamic model is developed of which it follows that saturation value of solute segregated in grain boundary, provided all available sites occupied, must be close to the solute concentration, in the nearest in composition phase in grain as it follows from phase diagram. Based on the analysis of experimental data for three binary systems (Cu –Sb, Fe – P and Fe – Sn) it is shown that the surplus concentration of the solute atoms in GB as compared with a concentration in grain is distributed equally between two fracture surfaces but takes up more than one atomic layer. The thickness of the segregation region (with surplus concentration in grain boundary) averages usually from 3 to 6 interatomic distances. As a rule, solute distribution is symmetrical on each side of the fracture surface. It is shown that maximum of grain boundary concentration falls on the first layer on the fracture surface. The method is developed of the calculation of the first layer concentration. It is shown that this concentration is close to the solute concentration in the nearest in composition phase in grain.