Abstract Relationships between synthesis conditions and structural or superconducting properties are discussed for simple cuprates (YBa 2Cu 3O 7, YBa 2Cu 4O 8 or CaBa 2Cu 3O 6) in the subperoxide model. It is shown that overoxidation (Ox) below a characteristic broad thermodynamic threshold (Δ H*∼200 kJ), induces metastable subperoxide (O q 1−2 q ) formation which can relieve structural slab mismatch. The thermodynamic parentage of subperoxides in peroxides (e.g., BaO 2 and fictitious CuO 2) is discussed; their similar thermodynamics can lead to ‘balanced’ subperoxides and with it superconductivity. By comparison the observation of strong c axis contraction and T c collapse (Col) is correlated with preferential occupation of O − on the apical sites (BaO 2). Reductive self doping (Red) of nominal semiconductors to O q 1−2 q can also occur due to the developing structural slab tension (tension doping), e.g., on cooling and superconductivity can be generally seen as the final step in a sequence of metastable escape routes. Accordingly subperoxidic electronic melting can be employed in order to adapt to spatial stress. Superconductivity is therefore primarily a crystal chemical problem with the creation and annihilation of subperoxidic O dictated by factors such as lattice pressure. The present work supplies a thermodynamic frame for these ideas.