New electrical conductivity and dielectric data for D–T gas at room temperature and densities of 10 to 120 mol/m3at 1592 Hz are reported. These plus earlier cryogenic gas and liquid data are reinterpreted in terms of space charging, which is present in almost all samples. This effect concentrates electric charge near the plates to cause an excess dielectric constant, and it also diminishes electrical conductivity by cancelling much of the external electric field. The mobilities of electrons and ions, plus their recombination coefficients, are included in the model. The effect of the 1592 Hz signal is also considered. Gas phase results are consistent with the presence of electrons, but the liquid may contain both electrons and negative ions.