Structural and electronic characterizations of (Ca 0.67Sr 0.33)Fe 2As 2 have been performed as a function of pressure up to 12 GPa using conventional and designer diamond anvil cells. The compound (Ca 0.67Sr 0.33)Fe 2As 2 behaves intermediately between its end members, displaying a suppression of magnetism and the onset of superconductivity. Like other members of the AFe 2As 2 family, (Ca 0.67Sr 0.33)Fe 2As 2 undergoes a pressure-induced isostructural volume collapse, which we associate with the development of As-As bonding across the mirror plane of the structure. This collapsed tetragonal phase abruptly cuts off the magnetic state and supports superconductivity with a maximum T c=22.2K. The maximum T c of the superconducting phase is not strongly correlated with any structural parameter, but its proximity to the abrupt suppression of magnetism as well as the volume-collapse transition suggests that magnetic interactions and structural inhomogeneity may play a role in its development. © 2012 American Physical Society.