We investigated the formation of extended defects in CaFeO2.5, predominantly appearing as antiphase boundaries (APBs), as a function of the synthesis method and temperature. While CaFeO2.5 is known to adopt an ordered oxygen defect structure showing long range order of the (FeO 4)∞ chains in its bulk form, interestingly, we demonstrated that the length of these (FeO4)∞ chains can be considerably scaled down to few nanometers by adopting a modified sol-gel method (low temperature synthesis) while the grain size of the resulting nano-phase CaFeO2.5 is around 50 nm. We discuss the synthesis dependent modulation of the length of APBs, characterized by X-ray diffraction and high resolution TEM, to be at the origin of an amplified switching dynamics of the (FeO4)∞ chains. This can accordingly explain the reduction of the onset temperature for oxygen diffusion to set in from 450 °C for bulk-CaFeO2.5 to 320 °C for nano-CaFeO2.5, as determined by 18O/16O oxygen isotope exchange reactions. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.