Raman spectroscopy is used to evidence both the nature of the interphase reaction between ZnO and MnO{sub 2} particles and its kinetic evolution. Zn cations migrate from the ZnO grains during oxygen vacancies formation process and diffuse into the MnO{sub 2} particles leading to an interphase region with an intermediate valence Mn{sup +3}-O-Mn{sup +4}. Large amounts of desorbed Zn cations promote the formation of ZnMn{sub 2}O{sub 4} structure, in addition to the intermediate valence state. The system evolves towards complete formation of the spinel phase at higher thermal treatment times. The reactivity of the ZnO plays an important role in the formation of this interphase. Low-reactivity ZnO powder, in which the oxygen vacancies are previously produced, shows a stabilization of the intermediate valence state with very limited formation of the spinel phase. A clear correlation between the amount of the intermediate state interphase and the magnetic properties has been established. - Graphical abstract: Recently new room temperature interphase magnetism has been reported to appear in ZnO-MnO{sub 2} system. Raman spectroscopy is used to evidence both the nature of the interphase reaction and the kinetic. The interphase evolved towards complete formation of the spinel phase. The reactivity of the ZnO plays an important role in the formation of this interphase. Finally, a clear correlation between the amount of the intermediate valence state and the interphase magnetic properties has been established.