We have investigated the thermochromic order‐disorder phase transitions in two superionic conducting compounds, Ag2HgI4and Cu2HgI4, using capacitance dilatometry to determine the relative volume change, &Dgr;V/V, at the transitions. We find an average &Dgr;V/Vof 9.6×10−3for Ag2HgI4at a transition temperature of 48.75 °C and an average &Dgr;V/Vof 4.0×10−3for Cu2HgI4at a transition temperature of 63 °C. ©1995 American Institute of Physics.