The structure and dielectric properties of the so-called M-phase solid solutions in the Li2O-Nb2O5-TiO2 system have been investigated. Detailed studies of the lattice parameters of these phases agree well with structure models based on intergrowths of LiNbO3 slabs with a titanium-rich corundum-type layer. The relative permittivity ranges from ~80 to ~55; microwave quality factors increase with the titanium content, reaching values of Q x f = 9000 at 6 GHz. The temperature coefficient of the resonant frequency changes sign within the solid solution region, thus permitting tunability to a zero value. All the compositions can be sintered to high density at temperatures ≤ 1100∘C.