The PbO-NiO-SeO2 ternary system was fully studied using constant hydrothermal Conditions at 473 K. It yields the establishment of the corresponding phase diagram using a systematic assignment of reaction products by both powder and single-crystal X-ray diffraction. It leads to the preparation of three novel lead nickel selenites, alpha-PbNi(SeO3)(2) (I), beta-PbNi(SeO3)(2) (II), and PbNi2(SeO2OH)(2)(SeO3)(2) (III), and one novel lead cobalt selenite, alpha-PbCo(SeO3)(2) (IV), which have been structurally characterized. The crystal structures of the a-forms I, IV, and III are based on a 3D Complex nickel selenite frameworks, whereas the beta-PbNi(SeO3)(2) modification (II) consists of nickel selenite sheets stacked in a noncentrosymmetric structure, second-harmonic generation active. The pH value of the starting solution was shown to play an essential role in the reactive processes. Magnetic measurements of I, III, and IV are discussed.