A new V IV /V V mixed-valence polyoxovanadate compound Ba 6 [V 10 O 30 (H 2 O)] ' 2.5H 2 O has been hydrothermally synthesized in an NaVO 3 +BaI 2 +(CH 3 ) 4 NI aqueous solution system. The compound crystallizes cubic, Im3 , a ؍ 16.8323(2) A s , and Z ؍ 6. The structural analysis gave wR2 ؍ 0.0597 for all of the unique 2641 re6ections and 83 parameters and R1 ؍ 0.0229 for 2217 re6ections with F O > 4 (F O ). The phase contains discrete[V 10 O 30 (H 2 O)] 12؊ polyoxovanadate ion and three types of Ba 2؉ cations, two of which connect the polyoxovanadate ion three-dimensionally. The remaining Ba 2؉ cation is attached to the discrete ion and is hydrated. The polyoxovanadate ion contains four V V 2 O 7 units of corner-sharing tetrahedral VO 4 pairs and one V IV 2 O 10 (H 2 O) unit. The two types of units are linked by corner-sharing. The V IV 2 O 10 (H 2 O) unit shows two types of unusual V+O polyhedral arrangements. One is a couple of VO 5 (H 2 O) octahedra sharing the water molecule, and the other consists of a VO 6 octahedron and a VO 5 square pyramid sharing a corner oxygen and includes a distant water molecule. Ba 6 [V 10 O 30 (H 2 O)] ' 2.5H 2 O exhibits Curie+Weiss-like paramagnetic behavior without any magnetic transition down to 5 K, probably re6ecting the discrete location of the polyoxovanadate ion.