An aqueous synthesis, involving the reduction of the VO₃⁻ anion in a mild alkaline pH in the presence of α-Bi₂O₃, led to the formation of a fully reduced polyoxovanadate (POV) capsule, with CO₃²⁻ anion encapsulation in its internal cavity, in the compound [Na₆(H₂O)₂₄][H₈V^IV₁₅O₃₆(CO₃)]·3N₂H₄·10H₂O (1). This CO₃²⁻ anion encapsulation, the source of which is absorbed aerial CO₂ in the pertinent aqueous alkaline reaction mixture, occurs only in the presence of α-Bi₂O₃. Compound 1 crystals, upon exposure to HCl acid vapor, exclude CO₂ gas that can react with the Grignard reagent (PhMgBr) to form triphenylcarbinol and benzoic acid; during this solid–vapor interface reaction, compound 1 itself transforms into an amorphous material that includes the Cl⁻ anion but could not be characterized unambiguously. Thus, we have synthesized a chloride ion (Cl⁻) encapsulated compound [Na₁₀(H₂O)₂₄][H₃V^IV₁₅O₃₆(Cl)]·6H₂O (2) in a direct synthesis protocol, which has been characterized by crystallography as well as by other spectroscopic methods. Compounds 1 and 2, each having fifteen vanadium(IV) centers, exhibit interesting magnetism in their solid states. The temperature-dependent magnetic susceptibilities for compounds 1 and 2 have been recordred at 0.1 T in the temperature range of 3–300 K. The temperature-dependent magnetic susceptibilities of compounds 1 and 2 are shown in the form of χ_Mvs. T and their product χ_MT vs. T plots.