A series of quaternary alkyl halide salts of 1,4-diazabicyclo[2.2.2]octane (DABCO) exhibited first-order phase transitions in the temperature range of 62–100 °C Abrupt changes of halide anion conductivities were observed by two to three orders of magnitude at the transition temperatures. The transition temperature and the heat of transition tend to increase with the increase of the alkyl chain length