In the Mg 4− x Ni x Nb 2 O 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 P3¯c1); the second one adopts the II-Ni 4 Nb 2 O 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 3 Nb 2 O 9 crystals identify the presence of planar defects and the intergrowth of several (structurally related) phases. The magnetic susceptibility of Mg 3 NiNb 2 O 9 , measured in the temperature range T=2–300K, shows no indications of magnetic ordering at low temperatures, while for MgNi 3 Nb 2 O 9 there is a magnetic ordering at temperatures below 45.5K.