Unknown thermodynamic data of a number of americium oxides were estimated. Phase diagrams of related binary and ternary systems in the Am–Mg–O system were calculated by transposing solution model parameters of the Pu–Mg–O system to the Am–Mg–O system. According to the calculated results, the liquid phase in the MgO–AmO 2− x system starts to form at 1930 K in the oxygen pressure range 4.3×10 −7 to 4.5 bar, and at the temperature varying from 2221 K to 2356 K at an oxygen pressure higher than 4.5 bar.