Molecular dynamics simulations and complementary neutron and x-ray diffraction studies have been carried out within the single phase glass forming range of (Y(2)O(3))(x)(Al(2)O(3))((100-x)), for x = 27 and 30. For x = 27, the experimental Al-O and Y-O coordination numbers are found to be 4.9 ± 0.2 and 6.9 ± 0.4 respectively, compared to 4.4 and 6.8 obtained from the simulation. Similar results were found for x = 30. An R-factor analysis showed that the simulation models agreed to within ∼6% of the diffraction data in both cases. The Al-O polyhedra are dominated by fourfold and fivefold species and the Y-O local coordinations are dominated by sixfold, sevenfold and eightfold polyhedra. Analysis of the oxygen environments reveals a large number of combinations, which explains the high entropy of single phase yttrium aluminate glasses and melts. Of these, the largest variation between x = 27 and 30 is found in the number of aluminum oxygen triclusters (oxygens bonded to three Al) and oxygens surrounded by three Y and a single Al. The most abundant connections are between the AlO(x) and YO(y) polyhedra of which 30% are edge shared. The majority of AlO(x)-AlO(x) connections were found to be corner shared.