Nonionic surfactants containing polyoxyethylene chains can act both as templates and as reducing agents in the synthesis of nanoparticles of noble metals from a solution of the metal salt. In this paper, we show that nanoparticles of platinum can be obtained by mixing one microemulsion containing a water-soluble platinum complex, [PtCl₆]²⁻, with another microemulsion containing a reducing agent, such as sodium borohydride (NaBH₄). The choice of surfactant is decisive in controlling the reaction rate. Whereas an alcohol ethoxylate gives a fast reaction regardless of the hydrophilic-lipophilic balance of the surfactant, reaction in a microemulsion based on the anionic surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT) was relatively sluggish. The difference is attributed to the nonionic surfactant assisting NaBH₄ as reducing agent. We also show that silver nanoparticles can be produced by reduction of a silver nitrate solution with a nonionic surfactant, and a block copolymer of the polyoxyethylene-polyoxypropylene-polyoxyethylene type, without the use of any additional reducing agent. This reaction takes place in the narrow channels of a reverse-hexagonal liquid- crystalline phase and the small silver particles became aligned into millimetre long fibres.