Using cold-seal hydrothermal bomb and piston-cylinder apparatus, we have carried out both forward and reversal experiments to investigate the phase boundary between nahcolite (NaHCO3) and trona (NaHCO3·Na2CO3·2H2O). We found that the temperature of this phase boundary remains low at least up to 10 kbar, so that this phase transformation maintains its univariant nature in our investigated P-T space. The locus of this phase boundary in a log(pCO2)-T space is defined as log(pCO2) = 0.0240(±0.0001)T − 9.80(±0.06) (with pCO2 in bar and T in K), in excellent agreement with earlier 1 atm experiments at different partial pressures of CO2 (pCO2) and theoretical calculation. Using this equation and literature thermodynamic data, the entropy of trona at 298.15 K is constrained to be 303.8 J mol−1 K−1, essentially identical to earlier estimates from different methods. Our experimental results have also been used to constrain the genesis of nahcolite in some fluid inclusions of diverse origins, and it is suggested that nahcolite in these occurrences is most likely a daughter mineral which crystallized from the fluids as temperature decreased, rather than an accidentally trapped phase.