The densities and heat capacities of aqueous solutions of boric acid have been measured at temperatures from 298.15 K to 705 K and at pressures up to 35 MPa. The results are in reasonable agreement with the previous measurements at temperatures of 600 K and below. At temperatures and pressures in the neighborhood of the critical point of water, the results show the expected dependence of partial molar volume and partial molar heat capacity of H3BO3(aq) on temperature and density. The boric acid molecules have strong enough attractive interactions with water that the apparent molar volume and the apparent molar heat capacity of H3BO3(aq) show behavior with the same sign as of aqueous NaCl, although the magnitude is much less.