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The magnetic structures of the two bismuth oxy-phosphate compounds BiMPO5 (M2+ = Ni2+, Co2+) have been determined by neutron powder diffraction using group theory analysis as a preliminary tool. Both compounds adopt a monoclinic crystal structure (S.G. P 21/n, a = 7.1642(2) A, b = 11.2038(3) A, c = 5.1740(2) A and β = 107.296(2)° for Ni2+ and a = 7.2441(1) A, b = 11.2828(1) A, c = 5.2258(1) A and β = 107.841(1)° for Co2+). The refinement of the magnetic structures below TN = 17.5 and 15 K, respectively, for both compounds show that the magnetic structure is characterized by the propagation vector k = (−1/2, 0, 1/2), with components given with respect to the reciprocal lattice of the nuclear structure. This means a magnetic unit cell that is a multiple of the nuclear cell. The magnetic structure is constituted of ferromagnetic pairs of metal ions antiferromagnetically coupled within double chains. The relative strength of the intra and inter double chains exchange interactions has been examined by establishing a theoretical magnetic phase diagram. Most of the interactions come from M–O–O–M super–super-exchange paths. At its ground state, BiNiPO5 shows a nearly collinear arrangement of magnetic moments with m1.5 K = 2.13(3) μB/Ni. Due to the strong magnetic anisotropy of Co2+ (m1.5 K = 3.52(3) μB/Co), the collinear character is largely lost while the magnetic structure remains analysable on the basis of the greatest isotropic component of the local moments.