The stability of titanium oxide (rutile) in aqueous sodium phosphates under hydrothermal conditions has been investigated. Hydrothermal methods to synthesize the titanium hideout reaction products were developed. The major product was identified as sodium titanium hydroxyphosphate (STHP), Na 4Ti(OH)2(PO4)2(s). The standard molar heat capacity of STHP, 'C'p0 (STHP,s) was measured for the first time by DSC. The enthalpy of solution of STHP in aqueous solution of perchloric acid and Na3NTA was measured in an isothermal microcalorimeter and used to calculate the standard molar enthalpy of formation of STHP, [Delta]f'H'0 (STHP,s) and new value for [Delta]f'H'0 (TiO2+,aq). Solubility measurements were carried out for STHP in equilibrium with TiO2(s) and this allowed the measurement of the Gibbs energy of formation for STHP, [Delta]f'G' 0 (STHP,s). The results were used to derive a thermodynamic model for the hideout reactions of titanium oxides and aqueous sodium phosphates in high temperature water.