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A few years ago we developed a new kinetic method for following transition-metal nanocluster formation in which the resultant nanocluster's catalytic activity was used as a reporter reaction via the pseudoelementary step concept. This method in turn yielded insights into a new, broadly applicable mechanism of nanocluster formation under H₂ consisting of (a) slow, continuous nucleation, A → B, followed by (b) fast autocatalytic surface growth, A + B → 2B (A = the nanocluster precursor, [Bu₄N]₅Na₃[(1,5-COD)Ir·P₂W₁₅Nb₃O₆₂], B = the resultant nanocluster's surface metal atoms), in which the nanocluster behaves as a “living metal polymer”. Herein, this new kinetic method is investigated and tested further:  (i) by following the Ir(0)_∼300 nanocluster's kinetics of formation more directly via the H₂ uptake reaction of the [Bu₄N]₅Na₃[(1,5-COD)Ir·P₂W₁₅Nb₃O₆₂] precursordoes this also show an autocatalytic H₂ uptake …