Enthalpies of solution of TiCl4(l) in aqueous perchloric acid have been measured in an isothermal calorimeter at T = 298.15 K at ionic strengths of (1.964, 3.002, and 4.062) mol center dot kg(-1). These results were extrapolated to zero ionic strength using an extended Debye-Huckel equation, to yield the standard enthalpy of solution Delta H-sln(m)degrees (T = 298.15 K) = - (268.03 +/- 0.45) kJ center dot mol(-1); from which the standard partial molar enthalpy of formation of the titanyl ion was derived: Delta H-f(m)degrees (TiO2+, aq, T = 298.15 K) = -(689.4 +/- 4.3) kJ center dot mol(-1). (c) 2006 Elsevier Ltd. All rights reserved.