This paper reports two new cobalt phosphonate compounds [NH3(CH2)(n)NH3]Co-2(hedpH)(2).2H(2)O [n = 4 (1), 5 (2)], where hedp is (1-hydroxyethylidene)diphosphonate [CH3C(OH)(PO3)(2)]. Both contain ladderlike chains with the same composition {Co-2(hedpH)(2)}(n)(2n-), where edge-shared {CoO6} octahedra are each bridged by O-P-O units. The chains are linked by very strong hydrogen bonds to form a three-dimensional open network with channels in which the diammonium counterions and lattice water reside. Magnetic studies reveal dominant antiferromagnetic interactions between the Co centers within the chain in both compounds, The field dependent magnetization confirms the occurrence of the field-induced magnetic transitions, with the critical fields ca. 25 kOe for 1 and 27.5 kOe for 2, respectively. In the case of compound 2, weak ferromagnetism is also observed at very low temperatures, possibly arising from spin canting of the antiferromagnetically coupled Co(II) ions in the chain. Crystal data: 1, monoclinic, P2(1)/c, a = 5.4868(8), b = 12.9116(18), and c = 15.251(2) Angstrom, beta = 98.843(2)degrees, V = 1067.6(3) Angstrom(3), Z = 2; 2, monoclinic, P2(1)/c, a = 5.4757(7), b = 12.7740(16), and c = 15.794(2) Angstrom, beta = 98.797(2)degrees, V = 1091.7(2) Angstrom(3), Z = 2.