In the Mg 4-xNi xNb 2O 9 (0⩽ x⩽4) system two ranges of solid solution have been found. One of the solid solutions has a corundum-related structure type (space group P3̄c1); the second one adopts the II-Ni 4Nb 2O 9 structure type (space group Pbcn). The unit cell constants and atomic positions have been determined and refined using neutron powder diffraction data. Electron diffraction and high-resolution transmission electron microscopy (HRTEM) from MgNi 3Nb 2O 9 crystals identify the presence of planar defects and the intergrowth of several (structurally related) phases. The magnetic susceptibility of Mg 3NiNb 2O 9, measured in the temperature range T=2-300 K, shows no indications of magnetic ordering at low temperatures, while for MgNi 3Nb 2O 9 there is a magnetic ordering at temperatures below 45.5 K.