The structure and conductivity of the new Li- and Mg-codoped bismuth niobates Bi1.5Mg1-xLixNb1.5O7-delta (0 <= x <= 0.50) with the pyrochlore structure have been investigated. The samples were synthesized by the method of organic-inorganic precursors combustion. A structural characterization was performed using Li-7 nuclear magnetic resonance (NMR) spectroscopy in combination with the fitting of X-ray diffraction patterns. The Li+ cations dynamics were studied by temperature-dependent Li-7 NMR lineshape analysis. The measurements have shown that the Li+ cations are distributed in one of two possible sublattices in the structure and are not mobile up to 120 degrees C. According to the results of structural analysis of the Bi1.5Mg1-xLixNb1.5O7 (x = 0.25; 0.50) pyrochlores the lithium atoms are distributed in the bismuth sites. The electrical properties were investigated by impedance spectroscopy method in the air, oxygen and "wet" atmospheres in the 25-750 degrees C temperature range. The activation energy value of dc conductivity is about 1.2-1.3 eV for the all samples and corresponds to the ionic (oxygen) conductivity at T > 400 degrees C. Electronic (p-type) conductivity was determined at T < 360 degrees C. H-1 MAS NMR data and the results of the comparison of the conductivity of Li- and Mg-codoped bismuth niobates in dry and "wet" atmospheres point to the proton conductivity up to 500 degrees C. The dielectric permittivity epsilon' values increase with lithium content from 86 (x = 0) to 143 (x = 0.5) at the same dielectric loss tan delta = 0.002 (1 MHz, 25 degrees C), TCC values vary from -590 to -530 ppm/degrees C in the 25-280 degrees C temperature range.