Co 1.7 Mn 0.3 (OH)PO 4 has been prepared under mild hydrothermal conditions and characterized from powder X-ray diffraction. IR data are characteristic of three distinct features corresponding principally to the vibrations of the hydroxide and [PO 4 ] 3Ϫ anions. UV-Visible studies show the existence of bands ascribed to the chromophores of Co ions in two coordination polyhedra, octahedral and trigonal bipyramidal. The magnetic behaviour indicates the existence of antiferromagnetic interactions as predominant. Magnetization measurements show the presence of two maxima at 65 K and 9 K, respectively. The first peak was attributed to a three-dimensional antiferromagnetic ordering and the second one reveals the possible existence of a spin-glass like state. The substitution of Co 2ϩ (S ϭ 3/2) by Mn 2ϩ (S ϭ 5/2) ions at about 15 % keeps the two magnetic transitions observed in the non-sub-1847 stituted phase. ZFC-FC curves show magnetic irreversibility at temperatures just below T N due to the spin decompensations in Co 1.7 Mn 0.3 (OH)PO 4 . The remanent magnetization indicates higher ferromagnetic interactions at lower temperatures than that observed in the ordered Co 2 (OH)PO 4 phase. These results could be attributed to the existence of a spin-glass state similar to that exhibited by the Co 2 (OH)PO 4 and substituted Co 2Ϫx M x (OH)PO 4 (M ϭ Ni, Cu; x ϭ 0.1Ϫ0.3) compounds. A comparative magnetic study with the analogous Ni 2ϩ (S ϭ 1) and Cu 2ϩ (S ϭ 1/2) phases is carried out.