Co 2 (OH)(PO 4 ) has been prepared from hydrothermal synthesis and characterized from powder x-ray diffraction. The nuclear and magnetic structures have been determined by neutron ͑D2B and D1B͒ diffraction data. The structure consists of a three-dimensional framework in which Co(1)O 5 -trigonal bipyramid dimers and Co(2)O 6 -octahedra chains are simultaneously present. The EPR spectrum of Zn 2 (OH)(PO 4 ):0.1%Co at 4.2 K shows a strong anisotropy of the g factor. The values obtained for the g tensor and the hyperfine coupling constants for the octahedral symmetry were g 1 ϭ5.890, g 2 ϭ4.550, and g 3 ϭ2.021 and A 1 ϭ240 ϫ10 Ϫ4 cm Ϫ1 , A 2 ϭ155ϫ10 Ϫ4 cm Ϫ1 , and A 3 ϭ85ϫ10 Ϫ4 cm Ϫ1 . Signals corresponding to the fivecoordinated Co͑II͒ ions were also observed. Magnetization measurements show the presence of two maxima at circa 75 and 15 K, respectively. The first peak was attributed to a three-dimensional antiferromagnetic ordering and the second one reveals the existence of a spin-glass-like state. This state with a cooperative freezing was also confirmed by both ac susceptibility measurements and magnetic irreversibility observed in the zero-fieldcooled-field-cooled signals. From low-temperature neutron-diffraction data, antiferromagnetic ordering is established with an ordering temperature of 71 K. The propagation vector of the magnetic structure is k ϭ͓0,0,0͔. The magnetic moments at 1.7 K are ferromagnetically coupled between CoO 6 -octahedra chains and the Co 2 O 10 dimers in the z direction. The values obtained for the magnetic moments are: 3.39(7) B ͓Co͑1͔͒ and 3.84(5) B ͓Co͑2͔͒. The absence of any anomaly in both the specific heat and thermal evolution of the magnetic moments below ϳ20 K confirms the blocking process of a spin glass behavior. The crystal-field splitting of the Co 2ϩ ions causes a single ion anisotropy along the z ͑c-axis͒ direction, giving an Ising character in which the local spins from the Co͑1͒ dimers are frozen. A magnetic frustration in the Co͑1͒ magnetic moments is observed as due to the presence of antiferromagnetic interactions between Co͑2͒ neighbor chains. It is to note the existence of a Co(1)-O(3)(PO 3 )-Co(2) superexchange angle with a value of 107°that involves ferromagnetic couplings between chain and dimer neighbors ferromagnetically coupled. This exchange pathway together with the anisotropy and frustration could be the responsible of the spin glass behavior observed in the three-dimensional antiferromagnetic Co 2 (OH)(PO 4 ) ordered phase.