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We propose a new statistical physics model to study equilibrium solute segregation at grain boundaries and the resulting embrittlement effect. This low-temperature expansion model is general and efficient, and its parameters can be obtained from atomistic calculations. It is possible to take into account multiple species, multiple segregation sites with different segregation free energies, account for configurational entropy, grain radius and site competition between solutes. As an example, the model is then applied to the study of phosphorus and hydrogen co-segregation at Σ3 109. 5 ° 0 1 ̄ 1 111 twin boundaries in α -Fe, using energetic parameters from density-functional theory calculations. We show that P H interactions may lead to increased P segregation at grain boundaries and cause additional embrittlement compared to the case where P and H are considered separately. Unlabelled Image • General statistical physics model of interface segregation at the atomic scale • GB coverage decreases with decreasing grain radius (not accounted for in McLean's segregation model) • Asymmetric co-segregation effects between P and H: H stabilizes P at GBs but P does not affect H segregation [ABSTRACT FROM AUTHOR]