The reaction of Rb, Cs, or Tl carbonates with a solution of phosphorous acid gave crystalline acid phosphites RbH2PO3(I), CsH2PO3(II), and TlH2PO3(III). The crystal structures of the compounds were studied by a single-crystal X-ray diffraction analysis at 150 K: I, monoclinic system, a= 7.530(2) Å, b= 8.634(2) Å, c= 12.426(2) Å, β = 102.46(3)°, V= 788.8(3) Å3, Z= 8, space group P21/c, R1= 0.0409; II, monoclinic system, a= 7.930(2) Å, b= 8.929(2) Å, c= 13.163(3) Å, β = 104.84(3)°, V= 900.9(4) Å3, Z= 8, space group P21/c, R1= 0.0239; III, orthorhombic system, a= 6.603(1) Å, b= 6.785(1) Å, c= 8.836(2) Å, V= 395.9(1) Å3, Z= 4, space group Pna21, R1= 0.0350. The PHO3tretrahedra in structures I–IIIare joined via hydrogen bonds into infinite zigzag-like chains [HPHO3]n–n, which form layers alternating with layers of metal cations. The layers of anionic chains are wavelike in Iand IIand planar in III. Apparently, IIIis not isostructural to Ior IIdue to the fact that Tl(I) has a stereochemically active pair of electrons.