The reactivity of the Zn–Mn–O system prepared by conventional ceramic routes using ZnO and MnO₂ as starting materials and in thin film form by pulsed-laser deposition is described and correlated with the system's magnetic response. Structural analysis is performed using X-ray diffraction analysis, X-ray photoelectron spectroscopy and Raman spectroscopy. The ferromagnetic phase is determined to be due to the presence of both Mn⁺³ and Mn⁺⁴ ions at the Zn diffusion front of the manganese oxide grain. Thus, it is demonstrated that Mn is not incorporated into the ZnO lattice, but that Zn diffuses into manganese oxide grains. Zn diffusion delays the total manganese reduction, i.e., Mn⁺⁴→Mn⁺³. At the diffusion front both manganese ions coexist and their spins couple ferromagnetically through a double-exchange mechanism. This mechanism explains the origin of the room-temperature ferromagnetism that has been recently discovered in the Zn–Mn–O system, and suggests that it is a unique and promising material for spintronic devices.