The compounds Ba(2)CoSi(2)O7, 1, and BaCo2Si2O7, 2, were obtained by heating mixtures of BaCO3, CoCO3, and SiO2 in the proper stoichiometries to 1300 degrees C for 48 h. Both materials were characterized by single-crystal X-ray diffraction and magnetization measurements on powder and partially oriented single crystals. Each cobalt ion in 1 is surrounded by four oxygen atoms derived from the [Si2O7](6-) groups in the form of a distorted tetrahedron. The cobalt ions are bridged by O-Si-O and O-Si-O-Si-O groups from the [Si2O7](6-) ions in an extended two-dimensional arrangement. The barium ions are arranged in layers that run parallel to the Co[Si2O7] layers. In 2 each cobalt ion is also surrounded by an arrangement of four oxygen atoms derived from the Si2O7 groups in a distorted tetrahedral shape. These tetrahedra are linked into chains through the sharing of one of the oxygen atoms from the Si2O7 groups. The CoO4 chains are crosslinked via O-Si-O-Si-O bridges from the Si2O7 groups. The barium ions occupy channels that run parallel to the crystallographic c-axis. Curie-Weiss behaviors and temperature independent paramagnetisms (TIP) coexist in the ranges 40-300 K and 100-300 K for 1 and 2, respectively. For both compounds the values of the Curie constant and TIP are consistent with tetrahedral Co(II) ions having three unpaired electrons. While compound 1 remains paramagnetic down to 5 K, 2 undergoes a transition into a state having a field-induced weak ferromagnetic response at T-c = 21 K. The weak ferromagnetic properties below T,can be explained by spin canting and spontaneous domain formation. The canting angle is estimated to be about 5 degrees at 5 K. Interestingly, 2 could represent the first example among the tetrahedral Co(II) compounds with the transition temperature (T-c = 21 K) being higher than the zero-field splitting of the (4)A(2) electronic ground state. Crystal data : for 1, formula Ba2COSi2O7, a = 8.450(1) Angstrom, b = 10.729(1) Angstrom, c = 8.474(1) Angstrom, beta = 111.365(8)degrees, monoclinic, space group = C2/c, Z = 4; for 2, formula BaCo2Si2O7, a 7.2131(6) Angstrom, b = 12.781(1) Angstrom, c 13.762(1) Angstrom, beta = 90.299(8)degrees, monoclinic, space group = C2/c, Z = 8.