Lithium nitride chloride (Li9N2Cl3) is a fast solid lithium ion conductor. According to analytical data from X-ray diffraction, DSC, DTA and conductivities, we found that a phase transition could take place when Li9N2Cl3 is at 250 degrees C. Lithium solid electrolytes of Li9-xMxN2Cl3 (M = Na+, K+, Rb+, Cs+) were prepared. The ionic conductivities of the prepared samples were determined, from room temperature to 350 degrees C, using a PAR368-1 AC-impedance spectrometer, with frequency measurements varying from 4 Hz to 100 kHz. The experimental results showed that the ionic conductivities could be of the order of 10(-1) S cm(-1) at 350 degrees C, with low electronic conduction and a high thermodynamic decomposition voltage at room temperature. The conductivity of Li9-xMxN2Cl3 depends on the ionic radius of Na+, K+, Rb+ and Cs+ ions substituted for the Li+ ion. We studied the atomic transport process of lithium ion conductors of Li9-xMxN2Cl3 using X-ray diffraction and differential thermal analysis.