Water-methanol mixtures (with a CH3OH mole fraction, chi, ranging from 0 to 1) are investigated using Broadband Electrical Spectroscopy (BES). A relaxation mode is detected in the 2 - 20 GHz frequency range, ascribed to the collective reorientational relaxation of the molecules. The interpretation of the results is carried out with an innovative thermodynamic formalism, that is not based on the Eyring equation and allows to determine general thermodynamic parameters able to clarify the structure of the water-methanol mixtures. The thermodynamic formalism correlates: (i) the relaxation parameters determined by BES; with (ii) the local intermolecular interactions (i.e., hydrogen bonds, HBs, and van der Waals interactions) that are responsible for the formation of structurally heterogeneous aggregates at the molecular level in the mixture. In particular, the various types of HBs in the mixture play a crucial role to modulate the relative abundance of aggregates and their topology. The information obtained with the proposed thermodynamic formalism is also confirmed by the analysis of the BES relaxation parameters in the framework of the Frolich theory. Finally, the presence of heterogeneities at the subnanometric scale in the mixtures, that is consistent with the results derived by the proposed thermodynamic formalism, is further corroborated by the application of the Bruggeman equation. The latter allows for the determination the volume fraction of water and methanol aggregates as a function of chi and the temperature. (C) 2018 The Electrochemical Society.