Dielectric loss measurements of lead and cadmium doped cesium halides have been reported in the temperature range 90–180°C. One loss peak which is attributed to the jump of vacancy in the six equivalent nn positions, has been observed in the plot of tan δ versus frequency isothermals. The plot of log fm (fm being the peak frequency) against 1/T gives a straight line, the slope of which yields the value of activation energy for the jump of a bound cation vacancy. The activation energy thus obtained from dielectric loss data compares well with those obtained from conductivity measurments on these crystals. © 1972, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.