X-ray absorption fine structure (EXAFS) measurements on 0.10-m strontium-containing solutions from 25 to 300°C at saturated vapour pressure indicate that the first shell Sr 2+-oxygen (water) bond length decreases from 2.57 to 2.52 (±0.01) Å. Over the same temperature range, the number of coordinated first shell water molecules decreases from 8 to 6. Simulations of a 1.35-m SrCl 2 using both rigid (SPC) and flexible (BJH) water models over the range from 25 to 330°C also show a decrease in the Sr 2+-oxygen distance of 0.03 Å, which is independent of density. EXAFS measurements and molecular dynamics simulations of concentrated SrCl 2 solutions up to 270°C and 300°C, respectively, show the presence of polynuclear cluster molecules, further underlining the importance of such species in defining the solute chemistry of hydrothermal solutions in the earth's crust. These data represent the first direct spectroscopic confirmation of solute clustering in high temperature aqueous solutions of alkali metal or alkali earth metal salts.