Densities of aqueous solutions of Na2CO3 and NaHCO3 with molalities from 0.1 mol.kg(-1) to 1.0 mol.kg(-1) have been measured from the temperatures 298 K to 623 K and pressures up to 28 MPa using a vibrating-tube flow densimeter. The volumetric results for Na2CO3(aq) and NaHCO3(aq) are in good agreement with the literature data at low temperatures. At high temperatures, the treatment of results must include the hydrolysis of Na2CO3(aq), disproportionation of NaHCO3(aq), and association of carbonate and bicarbonate ions with sodium ion.