The first order difference method of neutron diffraction and isotopic substitution has been applied to the Ni2+ and Cl- ions in a 2 m nickel chloride heavy water solution at 100 degrees C and 1000 bar, and at 300 degrees C and 1000 bar. The results show that both the Ni2+ and the Cl- hydration shell become progressively weakened with increasing temperature. The hydration number of Ni2+ drops from similar to 6 under ambient conditions to similar to 5.4 at 100 degrees C and to similar to 4.5 at 300 degrees C. The hydration number of Cl- at 100 degrees C (6.9) is similar to the value under ambient conditions (6.4) and reduces to similar to 4.9 at 300 degrees C.