Powder ZnAl2O4 (gahnite) samples doped with 0–100 at% Co were obtained by a sol-gel technique. X-ray powder diffraction was used to characterize the samples. Gahnite samples are cubic with the normal spinel structure, space group Fd3̅m. Cobalt doping caused a nonuniform increase of unit-cell parameter. The structure of the gahnite samples was refined by the Rietveld method. The location of Co2+ was determined by EPR spectroscopy. Cobalt doping of gahnite induces the inverse spinel structure at only 4 at% Co, and the inversion parameter increases with Co2+ doping level. Metal-oxide distances in the (Al,Co)O6 octahedra dominantly influence the unit-cell parameter of Co-doped gahnite.