The microwave dielectric properties and the microstructures of (Mg1-xCox)(2)TiO4 ceramics prepared by the conventional solid-state route were investigated. Lattice parameters were also measured for specimens with different x. The formation of solid solution (Mg1-xCox)(2)TiO4 (x=0.02-0.1) was confirmed by the X-ray diffraction patterns, energy dispersive X-ray analysis, and the lattice parameters measured. By increasing x from 0 to 0.05, the Q x f of the specimen can be tremendously boosted from 150 000 GHz to a maximum of 286 000 GHz. A fine combination of microwave dielectric properties (epsilon(r)similar to 15.7, Q x f similar to 286 000 GHz at 10.4 GHz, tau(f)similar to-52.5 ppm/degrees C) was achieved for (Mg0.95Co0.05)(2)TiO4 ceramics sintered at 1390 degrees C for 4 h. Ilmenite-structured (Mg0.95Co0.05)TiO3 was detected as a second phase. The presence of the second phase would cause no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase. In addition, only a small deviation in the dielectric properties was monitored for specimens with x=0.04-0.05 at 1360 degrees-1420 degrees C. It not only provides a wide process window but also ensures an extremely reliable material proposed as a very promising dielectric for low-loss microwave and millimeter wave applications.