Isobaric phase equilibria in Er₂O₃−H₂O−CO₂ and Tm₂O₃−H₂O−CO₂ systems have been determined at 650 and 1300 bars and temperature range of 100–800°C. The equilibria depend on the mole fraction of CO₂ in the coexisting fluid. The stable phases: Ln(OH)₃, Ln₂(CO₃)₃3H₂O, Ln(OH)CO₃-orthorhombic, Ln₂O₂CO₃-hexagonal, LnOOH and Ln₂O₃-cubic are common to both the systems. Additional phases observed in the thulium system are Tm₂O(OH)₂CO₃ and Tm₆(OH)₄(CO₃)₇. Two other phases isolated are Tm₆O₂(OH)₈(CO₃)₃ and Tm₄(OH)₆(CO₃)₃ which are stabilised only in the presence of alkali impurities. Stability field of Ln(OH)₃ is limited to$$X_{CO_2 }< 0 \cdot 01$$.Tm(OH)CO₃ does not stabilise at low$$X_{CO_2 } $$; therefore TmOOH coexists with all the phases other than Tm₆(OH)₄(CO₃)₇. When$$X_{CO_2 } = 1$$, the stable phases are Tm₂O₂CO₃ and Tm₂O₃ in the order of increasing temperature. The normal carbonate, Tm₂(CO₃)₃·8H₂O has no stability at higher pressures.