Abstract: The factors influencing the thermal stability of a vanadium phosphate (H₃NCH₂CH₂NH₂)4 [V^Ⅲ(H₂O)₂(V^ⅣO)₈(OH)₄(HPO₄)₄(PO₄)₄(H₂O)₂ ]·3H₂O (V₉P₈-en for short) with open-framework were investigated by XPS. The thermal degradation of V9P8-en while heated under Ar and air atmospheres was studied, including the removal of organic templates (en), phase transition and vanadium oxidation states. It is shown that the structure of the compound collapses with decomposition of the organic template before 380 ℃, and the oxidation states of vanadium atoms become variable, i.e. vanadium atoms in VPO skeleton are reduced from V⁴⁺ to V³⁺. As viewed from crystal chemistry, the structure has to be rebuilt because the original coordination environments of V⁴⁺ (distorted octahedron or trigonal-bipyramid) cannot match ones of V³⁺ (regular octahedron). It is concluded that the rational control of pretreatment conditions to maintain the original oxidation states of vanadium will be helpful for the thermal stability of open framework vanadium phosphates.