Weberites and pyrochlores (A 2 B 2 O 7 ), both fluorite-related superstructures, are attractive dielectric ceramics due to their ability to accommodate diverse cations, thus allowing their properties to be tailored. This study focuses on the fundamental understanding of the structure-dielectric property relationships in fluorite-related oxides. Specifically, Ln 3 NbO 7 and Ln 2 (Ln 0.5 Nb 0.5 ) 2 O 7 (where the ionic radius of Ln is smaller than that of Ln) compounds are investigated. It has been previously shown that weberite-type Ln 3 NbO 7 exhibits a composition dependent dielectric relaxation above room temperature. It is here shown that a dielectric relaxation also occurs in La 2 (Ln 0.5 Nb 0.5 ) 2 O 7 (Ln = Yb 3+ , Er 3+ , and Dy 3+ ) compounds near or below -158 • C. The temperature, at which the maximum permittivity occurs, is different for different compositions (-132 • C for La 2 (Yb 0.5 Nb 0.5 ) 2 O 7 , -197 • C for La 2 (Er 0.5 Nb 0.5 ) 2 O 7 , and -187 • C for La 2 (Dy 0.5 Nb 0.5 ) 2 O 7 at 1 MHz) and is correlated with the distortion of the NbO 6 octahedra. The room temperature dielectric permittivity of all three compounds was measured to be between 40 and 50 at 1 MHz.