Li3-2xMxInBr6 (M=Mg, Ca, Sr, Ba; x=0-1.0) was synthesized and the cation substitution effect was investigated by means of X-ray diffraction, Li-7 and In-115 nuclear magnetic resonance (NMR), and AC conductivity measurements. In Li3-2xMgxInBr6, the lattice constants a and c increased with an increase of x, but b and beta did not show significant changes. In the high-temperature phase for Li3-2xMxInBr6 (M=Mg, Ca, Ba), the conductivity decreased with x, accompanied by a broadening of the In-115 NMR spectra. In Li3-2xSrxInBr6 (xless than or equal to0.7), the conductivity of the high-temperature phase also decreased with an increase of x, but LiSrInBr6 showed almost the same conductivity to Li3InBr6. On the other hand, the conductivity of the low-temperature phase in Li3-2xMgxInBr6 (xless than or equal to0.4) increased with x, associated with a narrowing of the Li-7 NMR spectra. (C) 2004 Elsevier B.V. All rights reserved.