A part of microscopic structure in aqueous HNO3, and H2SO4 solutions was directly observed as ionic clusters isolated from these aqueous solutions by means of a specially designed electrospray mass spectrometer. The difference in the hydration structure for these acids was partially visualized on the basis of the molecular composition in the observed ionic clusters. For aqueous HNO3 solutions, the protonated water clusters, H+-(H2O)(n): n = 1, 2, 3.... which have similar mass distribution to the inherent water clusters, were observed predominantly. This is in good agreement with microscopic picture that the protons released from HNO3 are hopping and delocalized among water clusters. Such a cluster structure was independent of the HNO3 concentrations of [HNO3] < 1 mol/dm(3). On the other hand, for aqueous H2SO4 solution, cluster structure was drastically changed with varying H2SO4 concentration. For diluted H2SO4 solution, clusters that consisted of only water molecules were mainly observed. This means that the protons released from H2SO4 are moving among the water clusters the same as aqueous HNO3 solutions. At higher H2SO4 concentrations of [H2SO4] > 0.1 mol/dm(3), however, the self-association H2SO4 clusters accompanying less than six water molecules, H+(H2SO4)p(H2O)(q): p = 1, 2, 3.... 1 less than or equal to q less than or equal to 6, were formed favorably. The protons released from H2SO4 are not hopping among water clusters, and they would be localized in the self-association H2SO4 clusters under high concentration of H2SO4. The observed concentration effect of the cluster structure was strongly related to the equivalent conductivity of these aqueous acid solutions, that is, the ionization efficiency of the acids in the aqueous solution was explained on the basis of the cluster structure.