Measurements of the equivalent conductance of sulphuric, nitric, and hydrofluoric acids were made in water – hydrogen peroxide mixtures by means of a direct-current method. It was found that in solvents containing a high percentage of hydrogen peroxide the conductivities of these acids were extremely low, and it is proposed that this results from a marked decrease in their degrees of dissociation. This proposal is supported by similar measurements made on the system water – strong acid – weak acid.