Studies are reported on the Hg\HgO\ZrO2(Y2O3)/NaOH(aq)\H-2(Pt) cell at temperatures ranging from 298.15 K (25 degrees C) to 723.15 K (450 degrees C) and at a pressure of 275 bar, to assess the viability of ceramic sensors for measuring Henry's constant for hydrogen in high subcritical (473.15 K < T < 647.30 K) and supercritical (T > 647.30 K) aqueous systems. Previous work has shown that this cell is thermodynamically well-behaved, in that the measured potential is in good agreement with that calculated from thermodynamic data for the cell components. Thus, the Hg\HgO\ZrO2(Y2O3)\H+,H2O ceramic membrane electrode, when combined with a platinum electrode, provides a convenient means of evaluating hydrogen activity and Henry's constant for hydrogen in high subcritical and supercritical aqueous systems. The apparent standard Gibbs energies of formation of H,(aq) at temperatures within the range 473 and 723 K and at a pressure of 275 bar have been obtained. The calculated Henry's constants for dissolved molecular hydrogen in pure water were compared with the available literature data, and good agreement was found over the entire temperature range explored in this work.