Annealing studies were conducted on bulk La0.7Ca0.3MnO3-delta to determine the sensitivity of its structural and magnetic properties to oxygenation conditions. Standard bulk sintering conditions, thin-film annealing conditions for obtaining good magnetoresistive properties, and a reducing anneal, which corresponded to the onset of phase decomposition, were conducted, The main phase formed was a face-centered (fcc) pseudocubic double-perovskite structure, with cell parameters of a approximate to 2a(p) approximate to 0.772 nm, where a(p) is the single-perovskite cubic cell parameter. A minor superstructure-body-centered pseudotetragonal, with lattice parameters of c = 4a(p) and a = root 2a(p)-was observed in samples with (3 -delta) < 3, A maximum of 20% of the superstructure was formed using the most reducing conditions. The superstructure had a lower critical temperature than the main phase and depressed ferromagnetic order.