Eight new barium methylphosphonates were prepared and described. In dependence on pH, either barium hydrogen methylphosphonates or barium methylphosphonates can be formed. In the case of barium methylphosphonates, BaCH3PO3·3H2O crystallizes from the solution at room temperature and BaCH3PO3·H2O at a temperature above 65 °C. On heating, these hydrates form two anhydrous barium methylphosphonates (α-BaCH3PO3 and β-BaCH3PO3) with the same composition but with a different structure. In a basic environment, barium hydrogen methylphosphonate monohydrate, Ba(CH3PO3H)2·H2O, transforms to BaCH3PO3·3H2O through an intermediate with the formula Ba2(CH3PO3H)2(CH3PO3)·4H2O. The reverse reaction, that is the reaction of BaCH3PO3·3H2O with methylphosphonic acid, proceeds to the intermediate only and hydrogen methylphosphonate is not formed. Acidic Ba(CH3PO3H)2·H2O is able to interact with basic amines and form stable intercalates with them. Structures of β-BaCH3PO3 (P21/c, a = 8.4501(6) Å, b = 7.2555(7) Å, c = 7.4604(8) Å, β = 99.837(8)°, Z = 4) and BaCH3PO3·H2O (P21/c, a = 20.5077(5) Å, b = 7.2175(2) Å, c = 7.4909(3) Å, β = 95.522(3)°, Z = 8) were solved from powder X-ray diffraction data. Both compounds are layered, and the layers are formed of two sheets of Ba atoms connected through oxygen atoms of the phosphonate groups. The methyl groups point towards the interlayer space. In the case of BaCH3PO3·H2O, the molecules of water are coordinated to the Ba atoms and are placed in the interlayer space among the methyl groups.