K-birnessite was synthesized by air oxidation of Mn(Ⅱ) under alkaline conditions for ion exchange experiment. Energy-dispersive spectroscopy showed that K-birnessite has better cation exchange capacities with Ba²⁺, Pb²⁺, NH⁺₄ and Ag⁺ than Li⁺, Cu²⁺, Zn²⁺, Ni²⁺, Mg²⁺, Ca²⁺, Na⁺ and Cd²⁺. X-ray powder diffraction and infrared spectra revealed that Li⁺, Cu²⁺, Ba²⁺, Zn²⁺, Ni²⁺, NH⁺₄, Pb²⁺, Mg²⁺, Ca²⁺, Na⁺ and Cd²⁺ exchanged birnessites preserved the interlayer structure with d spacing of about 0.7 nm except for Ag-birnessite, but this exchange process greatly changed the crystallinity or the ordering of cations in the interlayer and MnO₆ octahedra in the layers of birnessite. The thermal property of cation-exchanged birnessites examined by DSC-TGA curves indicated that the important phase transformation and structure evolution occurred in the ranges of 150～250℃ and 500～700℃. Cation-exchanged birnessites experienced a disorder state and recrystallization for new structural arrangement as the temperature increased. The nature of heated products depends on the nature of cations, especially the size of ion radius. Large cations of K⁺, Ba²⁺, Na⁺, Pb²⁺ and Ag⁺ with radius larger than or close to 0.1 nm gave rise to hollandite type (2×2) tunnel and even todorokite type (3×3) tunnel manganese oxides. Birnessites doped with small cations (<0.1 nm) of Mg²⁺, Ca²⁺, Cd²⁺ and Zn²⁺ cations decomposed into hausmannite and of Cu²⁺, Ni²⁺, Li⁺ and heat-labile NH⁺₄ cations decomposed into bixbyite.