We compare the microwave and low-frequency (LF) dielectric conductivity of natural and artificial ice as a function of impurity concentration and temperature. We find a linear dependence of conductivity on acid concentration that is independent of the type of acid. There appears to be no evidence of significant dielectric dispersions between LF and 10 GHz in acid-doped ice. The results are well fitted by a model in which concentrated liquid acid at three-grain boundaries forms a network, earlier proposed as an explanation for the DC conductivity of polar ice. In contrast, evidence from ice with sea salt impurity shows large discrepancies between the microwave response of low salinity sea ice, and both the LF and microwave responses of ice with salinities typical of meteoric ice. These discrepancies may be attributed to sea salt chloride being largely incorporated within the ice lattice in meteoric-type, while in higher-salinity ice most of the sea salt is contained in platelike inclusions of liquid brine. ¿ American Geophysical Union 1993