BaNiO3 crystal to BaNiO2 crystal transformation is reported. Contrary to an intuitive topochemical reduction, a two steps reaction was observed. In the first step, NiO exsolution occurs and intermediate Ba1+xNiO3 phases were obtained and isolated. A composite approach was used to solve the novel structure for x ~ 1/6 with charge ordered Ni 2+ and Ni 4+. We argue that this NiO exsolution is responsible for the increased oxygen enhanced reactivity recently reported. Upon re-oxidation, oxygen-defficient mixed valent BaN i3/4+ O3-x are obtained such that the full redox cycle is irreversible and goes through a diversity of structural and nickel valence adaptative oxides. The exsolution of metal during reduction of transition metal oxides is common in nickel based ABO3 compounds, and was recently observed on the hexagonal perovskite polytype of Ba8Ta6NiO24. 1 More rarely, structures based on oxo-anion frameworks can exsolve metal oxide upon oxidation. This results in complex depleted lattices after partial metal oxide removal, observed for instance in olivine-like LiFe(PO4), layered BaFe2(PO4)2 and more recently in the Dumortierite Fe13.5(AsO4)8 (OH)6 compounds, 2-4 the Fe 2+ /Fe 3+ redox couple leaves Fe-deficient compounds with α-Fe2O3 nano-particles decorating the surface. The possibility to exsolve other transition metal oxides seems appealing. However, a suitable crystal structure with the required metal redox capabilities (oxidation potential : Ni 2/3+ > Co 2/3+ > Mn 2/3+ > Fe 2/3+) is difficult, it often results in stable trivalent iron ions (Fe 3+) but divalent Ni 2+ , Co 2+ and Mn 2+ in oxides. To circumvent this stability, here we show an original path for NiO exsolution using moderate reduction of super-oxidized 2H-BaNi 4+ O3 into BaNi 2+ O2. The mechanism involves the appearance of nickel deficient 1D