The partitioning of Mg, Fe and Co between coexisting olivine and orthopyroxene found in peridotite inclusions is examined from the thermodynamic point of view, using a binary and a ternary regular solution models. The following two exchange reactions are considered: MgSi_0.5O₂+FeSiO₃ = FeSi_0.5O₂ +MgSiO₃ (1) and MgSi_0.5O₂+CoSiO₃=CoSi_0.5O₂+MgSiO₃. (2) The apparent partition coefficients are respectively: RT·lnK'_Fe.Mg=-ΔG₁-(2X_Mg⁰¹-1)(W⁰¹P+α⁰¹)+(2X^opx_Mg-1)(W^opxP+α^opx) (3) and RT·lnK'_Co.Mg=-ΔG₂+(Δα+ΔWP)X_Fe (4) Using (3) and (4), we can estimate the P-T conditions at equilibrium of peridotite inclusions, if we know the numerical values of the apparent partition coefficients. Examination of the equilibrium relations in a peridotite inclusion from a South African kimberlite and in an lchiomegata lherzolite leads to the conclusion that the garnet peridotite is formed at higher pressure and lower temperature than the lherzolite.