HeteroPolyAcids (HPA's) are a class of solid acids that have broad applications in many fields of science and technology, including catalysis and chemical engineering. The proton locations within the thermally stable and commonly known Keggin unit, which is the primary structure building unit/block, has remained undetermined in anhydrous HPAs, despite numerous theoretical and experimental efforts. However, Rotational Echo DOuble Resonance (REDOR) NMR and Density Functional Theory (DFT) quantum chemical calculations offer a new opportunity to determine the exact locations of protons within the Keggin unit. The crucial experimental evidence is provided for the basic and very extensively studied acidic form of H8-nXn+M12O40, X = Si, P and M = Mo, W, belonging to the Keggin structure. While showing that the acidic protons are located in the bridging oxygen positions (RP-H = 520 ± 20 pm) in H3PMo12O40 and in the terminal oxygen positions (RP-H = 570 ± 20 pm) in H3PW12O40, REDOR measurements also provide for the first time the structural basis to consistently rank the acid strength for the important class of Keggin solid catalysts.