Molar enthalpies of dilution Delta H-dil(m) of Na2CO3(aq) were measured from molality m = 1.45 mol . kg(-1) to m = 0.008 mol . kg(-1) at seven temperatures from T = 298 K to T = 523 K at the pressure p = 7 MPa, and at four temperatures from T = 371 K to T = 523 K at the pressure p = 40 MPa. Molar enthalpies of dilution Delta H-dil(m) Of NaHCO3(aq) were measured from m = 0.98 mol . kg(-1) to m = 0.007 mol . kg(-1) at the same temperatures and pressures. Hydrolysis and ionization equilibria contribute substantially to the measured enthalpies under many of the conditions of this study. Explicit consideration of these reactions, using thermodynamic quantities from previous studies, facilitates a quantitative representation of apparent molar enthalpies, activity coefficients, and osmotic coefficients with the Pitzer ion-interaction treatment over the ranges of temperature, pressure, and molality of the experiments.