Silver nanoclusters in varying amounts were introduced into the poly(2-vinylpyridine) (P2VP) phases of three types of poly(styrene-6-2-vinylpyridine) block copolymers. Upon casting the block copolymer-silver nitrate-water-1,4-dioxane solutions and drying, the silver clusters selectively formed in the P2VP phases of the microphase-separated films. From wide angle X-ray scattering measurements and transmission electron microscopy it was found that microspheres composed of polystyrene shells and P2VP cores containing silver nanoclusters were formed by crosslinking of the P2VP phases and reduction of the silver compounds in the films. The content of silver nanoclusters in the microspheres could be controlled quantitatively by this method.