Soluble crosslinked macromolecules (micro-gels) used as stabilizers of catalytically active metal nanoclusters allow control of the nanocluster size by simply changing the crosslinking degree of the microgel. Microgel-stabilized metal nanoclusters are easily prepared and stored in the solid state, and those of Pd (see Figure) exhibit high catalytic activity in the Heck reaction of activated aryl bromides.