The crystal and molecular structure of two novel nickel complexes stabilized by pentathionate counterions, namely acetato(O,O′)diaqua biquinoline(N,N′)nickel(II) hemipentathionate dihydrate (1) and diterpyridine (N,N′,N″)-nickel(II) pentathionate tetrahydrate (2), are presented. Compound (1) is monoclinic, C2/c, a 21.5628(4), b 11.3354(2), c 20.2873(4) Å, β 92.433(1)°, V 4954.2(2) ų, Z 8, conventional R (on F) being 0.042 for 3444 N_o observed reflections (I > 2σ(I)). Compound (2) is triclinic, P1^-, a 12.461(1), b 12.781(1), c 23.542(2) Å, α 80.246(5), β 79.714(5), γ 88.680(6)°, V 3635.9(6) ų, Z 4, R 0.062, N_o 6255. In both compounds the pentathionate anions act as counterions of the octahedral complex cations and have been formed in situ through reactions involving the thiosulfate group present in the synthesis process.