Abstract We report here on the silver (Ag) nanoparticle synthesis in aqueous solutions of poly(ethylene oxide)-poly(propylene oxide) (PEO–PPO) block copolymers in the absence of any additional agents. In particular, we examined the effect of reaction temperature, molecular weight of PEO–PPO block copolymer and PEO–PPO block copolymer concentration on the reduction of silver ions (Ag + ) and the resulting formation of Ag nanoparticles in aqueous PEO–PPO block copolymer solutions. We found that Ag nanoparticles were formed from aqueous silver nitrate (AgNO 3 ) solutions containing PEO–PPO block copolymer above 100 °C. This is most likely due to the dehydration of Ag + caused by thermal motion of water molecules in higher-temperature aqueous solution at high vapor pressure. We also found that the formation of Ag nanoparticles in aqueous PEO–PPO block copolymer solutions was enhanced with larger molecular weight of PEO–PPO block copolymer and with increase in the concentration of PEO–PPO block copolymer in aqueous solutions.