The electrocatalytic performance of a model series of Ni/Mo binary solid solutions for H-2 evolution in aqueous acid is compared with those of physical mixtures in pressed fine powder form. Synergy is expressed in the alloys down to remarkably low levels of Mo (< 5 at%), and to a significant extent also in physical mixtures. The synergy is best interpreted within the framework of a Volmer-Heyrovsky mechanism in which simple cooperative functioning of the components is mediated via rapid intra- and interparticle surface diffusion of H adatoms, i.e. the well-known ＇spillover＇ process in heterogeneous catalysis. In its fresh state, Ni effects initial proton discharge and acts as H-source for Mo, where ion/atom recombination (and H-2 desorption) is promoted more efficiently. Deactivation in the alloys is clearly linked with the Ni component and attributed tentatively to buildup of a subsurface Ni hydride. In its perceived role as surface II-trap and ultimate ＇sink＇, Mo also serves to protect the Ni from deactivation. The proposed model is simple, hinging mainly on established processes in surface chemistry and ＇demystifies＇ a technically important and long-standing phenomenon in electrocatalysis.