AbstractAn examination of the Na/V/P/N(C2H5)3/H2O system at 473 K under hydrothermal conditions yields several different phases in the pH range of 2.65 to 12. At low pH values a NaV4+OPO4phase was obtained. In the weakly acid, neutral or low basic pH ranges the Nav[((V 4−w4+V 1+w5+) O9)(PO4)2] · (PO4)x· (OH)y· zH2O (or FVP‐1 for short) phase was found (with v = 2.8 to 4.0, w = −0.1 to 1.1, x = 0 to 0.2, y = 0 to 2.1 and z = 7 to 10). Its crystal structure was solved in space group Im3m with a = 16.025 Å. It contains V 44+V5+O9(PO4)2groups in which the vanadium atoms have a square pyramidal coordination of oxygen atoms in the shape of a spiked helmet, where the spike is the vanadyl group of the central V5+coordination polyhedron. These groups are statistically arranged around a mirror plane yielding an V10/2O18/2(PO4)2grouping and connect to form an open porous framework by bridging via the phosphate groups following the topology of the NbO type net. The framework has a mean free channel diameter of about 4.1 Å and a density of 10.2 vanadium and phosphorus polyhedra per 1000 Å3. This makes it, together with two other recently found related frameworks, the most open microporous framework described so far. No aluminosilicates or aluminophosphates of similar openness are known. In air the FVP‐1 phase does not collapse when heated to 523 K, in vacuum it remains stable to 623 K. It dehydrates and rehydrates reversibly. Redox titrations of 19 samples show that the V4+/V5+ratios vary from 4.1(2)/0.9(2) to 2.9(2)/2.1(2). Syntheses using V2O5as a source of vanadium yield samples with lower V4+/V5+ratios than those based on VOSO4. The fact that FVP‐1 displays disorder makes the compound possibly interesting for potential c