The ternary isobaric (TX) prism for the system CaO–CO2–H2O was determined at 1,000 bars pressure between 600° C and 1,320° C. At this pressure, calcite melts incongruently at 1,310° C, portlandite (Ca(OH)2) melts congruently at 835° C, a binary eutectic exists between calcite and portlandite at 685° C, melting begins at 740° C on the join calcite-water and the univariant (isobaric invariant) equilibria lime³calcite³portlandite³liquid and calcite³portlandite³liquid³vapour occur at 683° C and 675° C, respectively. The latter is the minimum liquidus temperature in the TX prism, and the composition of this liquid is 65CaO, 19CO2, 16H2O (in weight per cent). PT curves were determined for several univariant equilibria. In the binary system CaO-H2O, four univariant curves meet at an invariant point, at 810° C and 100 bars pressure. Portlandite dissociates only at pressures below this point. The minimum liquidus temperature in the ternary system varies between 685° C and 640° C in the pressure interval 27 bars to 4, 000 bars.