An extraordinary high-speed synthesis of gold nanoparticles (AuNPs) was discovered by synthesizing the AuNPs in ethanol-water mixtures using a solution plasma process (SPP). The influence of the ethanol mole fraction (χethanol) in the ethanol-water mixtures on the reduction rate of gold chloride ions to AuNPs under the SPP system was studied. The results indicated that the reaction rate of the AuNPs synthesis exhibited a maximum value (i.e. 35.2 times faster than in a pure water system) at the significant point where the partial molar volumes of ethanol and water changed drastically.