The first order difference method of neutron diffraction and isotopic substitution has been applied to argon dissolved under pressure in heavy water. The results show that argon atoms possess a well-defined nearest neighbor hydration shell composed of 16(2) water molecules in the range 2.8 less than or equal to r (Angstrom) less than or equal to 5.4. Also evident is a weaker second shell which extends to : similar to 8 Angstrom. The results are in broad agreement with those obtained from Monte Carlo and M.D. computer simulation of models in which an argon atom interacts with a number of water molecules via pairwise additive forces. Significant differences are discernible when attempts are made to match quantitatively the theoretical results over the entire range of the experimental information.