Neutron diffraction experiments were carried out on a mixture of argon in water under a pressure of 300 bar and a temperature of 318 degreesC. The data were analyzed by standard procedures and showed that the argon atom is hydrated with 9 (1) water molecules, a substantial reduction from the value of 16 (2) at ambient temperature and a pressure of 250 bar. This result is in excellent agreement with the computer simulation study of Guissani and Guillot, which was carried out on an argon/water mixture at high temperatures and pressures.