Iron-rich Ruddlesden–Popper phases Sr3Fe2 −
≤ 1) have been synthesized by two citrate precursor techniques and investigated in order to explore the correlation between electron transport properties, electronic state, magnetism, and magnetoresistance effects in iron(IV) based oxides. The materials were characterized by X-ray powder diffraction, temperature and field dependent magnetization measurements, magnetotransport studies, and iron-57 Mössbauer spectroscopy. With increasing cobalt content there is a smooth development of electronic properties: a gradual increase in the low-temperature conductivity of the semiconducting materials, a successive reduction and finally suppression of the charge ordering of Fe(4 −
Δq)+ and Fe(4 +
Δq)+ sites resulting
from a charge disproportionation of the Fe4+ ions, and an evolution from antiferromagnetic to ferromagnetic behaviour via a spin glass regime. Large magnetoresistance (MR) effects at low temperatures were apparent for all the samples (up to −47% at 5 K and 7 T). Maximization of the MR effects in the title system appears to involve an optimized balance between ferro- and antiferromagnetic interactions.