The Extended Brewer Interactive Interionic Bonding Theory (EBIIBT) has been developed to show the equivalence of interatomic and interionic bonding feature, as well as its effect upon electrocatalytic properties for the hydrogen electrode reactions. The equivalence of interionic hypo-hyper-d-interelectronic interaction in both metallic and any other ionic or their composite states has been proved and inferred. In such a context, the EBIIBT stays in the core of the Strong Metal-Support Interaction (SMSI) effect in heterogeneous catalysis, and the latter is an implied consequence of the former. Thermal Gravimetry (TG) analysis of Temperature Programmed Reduction (TPR) of mixed hypo-hyper-d-electronic oxides of transition elements was broadly employed to prove the extended Brewer theory along the phase diagrams of such constituents, as reflected in dramatically decreased temperatures of their mutual reduction versus the individual metallic ingredients entering into the resulting intermetallic phases or alloys. The same interionic Brewer (and/or intermetallic) bonding effect has also been confirmed both by Underpotential Deposition of hyper-d- upon hypo-d-electronic substrates and vice versa, and by the shift of the bonding peaks in XPS analysis. The former affords the basis for new trends in submonolayer hypo-hyper-d-interelectronic electrocatalysis of transition metals. Thus, it has been pointed out that the term SMSI has primarily a broader hypo-hyper-d-interelectronic interactive sense in both the bonding effectiveness and synergism in electrocatalysis, including bifunctional catalytic meaning as based upon the interionic interactive promotion. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.