Dielectric ceramics in the system (Zn1-xCox)TiO3 (x = 0-1) were synthesized by the solid-state reaction route. The phase distribution, microstructure, and dielectric properties were characterized by using powder X-ray diffraction analysis, electron microscopy, and microwave measurement techniques. Three phase composition regions were identified in the specimens sintered at 1150degreesC; [spinel + rutile] at 0 less than or equal to 5 x less than or equal to 0.5, k[spinel + ilmenite + rutile] at 0.5 < x less than or equal to 0.7, and [ilmenite] phase at 0.7 < x less than or equal to 1. For the 0 less than or equal to x less than or equal to 0.5 region, the amount of Ti-rich precipitates incorporated into the spinel phase decreased with the Co content at 0 less than or equal to x less than or equal to 0.5, with a concomitant increase of the rutile phase. The ilmenite phase appeared for high Co content. The microwave dielectric properties depended on the phase composition and volume according to the three phase regions, where the relative amount of rutile to the spinel or ilmenite determined the dielectric properties. The dielectric constant as a function of Co addition was modeled with a Maxwell mixing rule. An optimum phase distribution was determined in this system with dielectric constant of 25, a Q*f 70 000 GHz, and a low temperature coefficient of the resonant frequency.