The superlattice structure of 2,5-dihydroxythiophenol (DHTP) irreversibly adsorbed on Au(111) in 0.1 M perchloric acid was determined in situ based on dual resonant, rotational anisotropy, second harmonic generation (RA-SHG) measurements. Plots of the intensity of the SH Light as a function of the azimuthal angle displayed two clearly defined 3-fold pattern features. These curves were analyzed in terms of a model which regards the total interfacial nonlinear susceptibility as the sum of contributions due to the molecular overlayer itself and due to the attenuated substrate. A statistical fit to these data was found to be consistent with a (root 7 x root 7)R19.1 degrees superstructure for the DHTP/Au(111) adlayer for which the surface coverage (theta) is 1/7. This value of theta was found to be in excellent agreement with that calculated horn the normalized charge under the surface-confined two-electron hydroquinone/quinone voltammetric peaks observed for the same precise DHTP/Au(111) interface.