The microwave dielectric properties and microstructures of (Mg1-xCox)(1.8)Ti1.1O4 (x = 0.03-1.00) solid solutions prepared by the conventional solid-state route were investigated. Lattice parameters were also measured for the specimens with different x. The forming of complete (Mg1-xCox)(1.8)Ti1.1O4 solid solutions were confirmed by the X-ray diffraction patterns analysis, the measured lattice parameters, and the cell volume, which linearly varied from 300.82 angstrom(3) for x = 0.03-301.42 angstrom(3) for x = 1.00. By increasing x from 0.00 to 0.05, the Q X f of the specimen can be tremendously boosted from 141 000 GHz to a maximum of 207 500 GHz. A fine combination of microwave dielectric properties (epsilon(r) similar to 16.11, Q X f similar to 207 500 GHz at 10.72 GHz, tau(f) similar to -52.6 ppm/degrees C) were achieved for (Mg0.95Co0.05)(1.8)Ti1.1O4 ceramics sintered at 1390 degrees C for 4 h. Ilmenite-structured (Mg0.95Co0.05)TiO3 was detected as the second phase. The presence of this second phase did not cause any significant variation in the dielectric properties of the specimen. This may be because the second phase properties are also very similar to the main phase. Therefore, it is proposed as a very promising dielectric material for ultra-high-frequency applications.