We have previously reported on the preparation of calcium superoxide, Ca(O2 )2 , from calcium peroxide diperoxyhydrate, CaO2 ·2H2 O2 . In that work, the preparation of a product of higher purity than that reported elsewhere was ascribed to the carefully controlled reaction conditions. The effect of temperatures between 25° and 60°C (298–333 K), reaction time, and reactant packing were studied. In further experiments reported here, disproportionations at lower temperatures and a range of reaction chamber pressures were studied. Disproportionations at 0°C (273 K) and surface spreadings of 312 to 36 cm2 /g resulted in 59–61 percent purity Ca(O2 )2 , showing that higher surface loadings could be used in the reaction dish at subambient temperature than at ambient temperature. This is a significant feature for prospective scale up of the mass prepared in a single experiment. Reactions were studied at chamber pressures between 6 µm and 300 mm Hg (0.8 Pa–40 kPa) at 30°C (303 K). The optimum pressure for product purity was 90 µm Hg (12 Pa), possibly because the reactant overheated at lower pressures. Product purity at higher pressures improved when molecular sieve desiccant was mixed with the reactant.