The coefficients of the conductivity theory for the multicomponent system of Onsager-Fuoss-Kim-Chen-Quint-Viallard for aqueous HCl-H2SO4 mixtures have been computed. The experimental results for the mixtures of equal molarity and equal ionic strength of the same system agree well with the theory. The excess conductivity of mixing is calculated. The mixing coefficients of Friedman and of Timmermann are derived. These two sets of coefficients agree with those derived from the excess conductivity of mixing. The extrema of the mixing coefficients as predicted by Friedman are observed.