The electrical conductivities of fused AlI3, GaI3, InI3, BiI3, CdI2 and I2 have been measured, over the pressure range 0–1000 bar (0–100 MN m–2) at a series of temperatures. In every case, the conductivity increases with pressure, corresponding to a negative value of the “activation volume”ΔVκ=–RT(∂ ln k/∂P)T. ΔVκ is numerically greatest for those compounds having the lowest conductivity at atmospheric pressure (AlI3, GaI3), and is smaller for the more ionised melts (BiI3, InI3, CdI2). The large negative values of ΔVκ for AlI3 and GaI3(–ΔVκ > 100 cm3 mol–1) are attributed to electrostriction accompanying the formation of solvent shells around the ions when these are formed in the molecular “solvent”. The effect becomes less marked as the degree of ionisation of the melt increases. For some of the compounds studied, the conductivity decreases with rising temperature at atmospheric pressure, but, under constant density conditions, the conductivity increases with temperature in all cases for which (∂P/∂T)V is known. The decrease observed at atmospheric pressure is caused by a reduction in the degree of ionisation as the melt expands thermally.