The bimetallic origins of catalytic synergism were studied using unmodified rhodium and manganese carbonyls as catalyst precursors during the low-temperature hydroformylation of 3,3-dimethylbut- 1-ene to 4,4-dimethylpentanal in n-hexane solvent (T approximate to 298 K, P-CO = 1.0-4.0 MPa, P-H2 = 0.5-2.0 MPa). A dramatic increase in the catalytic rate was observed in the experiments conducted when both metals were used simultaneously. Detailed in-situ FTIR measurements indicated the observable presence of only homometallic complexes during catalysis, e.g., RCORh(CO)(4), Rh-4(CO)(12), Rh-6(CO)(16), HMn(CO)(5), and Mn-2(CO)(10). The kinetics of product formation show a distinct linear-bilinear form in observables: k(1)[RCORh(CO)(4)][CO](-1)[H-2] + k(2)[RCORh(CO)(4)][HMn(CO)(5)][CO](-1.5). The first term represents the classic unicyclic rhodium catalysis, while the second indicates a hydride attack on an acyl species. These spectroscopic and kinetic results strongly suggest that the origin of synergism is the presence of bimetallic catalytic binuclear elimination and not cluster catalysis. This appears to be the first detailed evidence for such a catalytic mechanism, and its implications for selectivity and nonlinear catalytic activity are accordingly discussed.