Calcium aluminate glasses are important materials where AlO(4/2) is the only network former. Aluminum in crystals or glasses between CaO and Al(2)O(3) can have different environments as a function of the CaO/Al(2)O(3) ratio. Using X-ray absorption at the Al K- and L-edges, Raman and (27)Al NMR spectroscopies, we have determined the structural surroundings of Al in glasses, crystals, and melts in this binary system. Aluminum is in octahedral coordination at high-Al(2)O(3) content (>80 mol%) and essentially in fourfold coordination with 4 bridging 0 atoms (BOs) at Al(2)O(3) contents between 30 and 75 mol%. At around 25 mol% Al(2)O(3), Al is in tetrahedral coordination with two BOs. The presence of higher-coordinated species at high-Al(2)O(3) contents and their absence at low Al(2)O(3) imply different viscous flow mechanisms for high- and low-concentration Al(2)O(3) networks.