© 2017 The Ceramic Society of Japan and the Korean Ceramic Society Crystal structures and electronic transformations of quadruple iron perovskite solid solution Ca 1−x Sr x Cu 3 Fe 4 O 12 (x = 0.2, 0.4, 0.6, and 0.8) have been investigated by synchrotrons X-ray powder diffraction, Mössbauer spectroscopy, and magnetization measurements. For x = 0.2, a charge disproportionation transition (2Fe 4+  → Fe 3+  + Fe 5+ ) occur simultaneously with electron charge transfers from Fe to Cu below ∼200 K, as well as CaCu 3 Fe 4 O 12 . In contrast, negative thermal expansions derived from continuous electron charge transfers from Cu and Fe are observed for x = 0.6 and 0.8 at low temperatures below room temperature, as in SrCu 3 Fe 4 O 12 , followed by charge disproportionation transitions. A two-phase coexistence is observed at low temperature below ∼200 K for x = 0.4, indicating that the phase boundary locates in the vicinity of this composition. We have discovered that the Fe–O bond lengths are closely related to their covalency which were estimated from Mössbauer isomer shift parameters. The Fe–O bond covalency plays a crucial role in the types of electronic phase transitions for the Ca 1−x Sr x Cu 3 Fe 4 O 12 and R 3+ Cu 3 Fe 4 O 12 (R: trivalent rare earth metal ions, Y, La–Lu) systems, where the two different low-temperature electronic phases are separated by a common isomer shift value of ∼0.17 mm s −1 .