Continuous synthesis of silver nanoparticles was successfully achieved using a novel counter-current pipe in pipe hydrothermal reactor. Aqueous solutions of silver acetate (99% Sigma Aldrich) were flowed in aqueous solutions with concentrations of 0.0005 M to 0.03 M. Supercritical conditions were shown to be necessary to produce detectable quantities of products. The conversion of the metal precursor in solution was found to be between 88% and 96% depending on concentration and reactor temperature. The role of Polyvinylpyrrolidone (PVP50) ((C6H9NO)(n) of M-w 50,000) was investigated either as an additive in the precursor flow or as an additive downstream (post-production). The PVP50 served three main functions: (1) prevention of nanoparticles adhering to the walls of the sample container, (2) it allowed higher concentrations of silver precursor to be used without silver particles sticking to the inside in the reactor and (3) it stabilised the silver particles in the product suspension. The PVP did not undergo significant oxidation or breakdown as a result of passing through the reactor due to the very short residence time at supercritical conditions (less than 2 s). From Transmission Electron Microsope (TEM) analysis, at 415 degrees C, the average particle size of the silver nanoparticles was found to be 3040 nm with a maximum particle size of 70 nm. Addition of PVP reduced the particle size distribution of the product. Increasing the temperature from 380 degrees C to 430 degrees C in the reaction zone also decreased the particle size. Slower flow rates at 380 degrees C resulted in larger particles being formed. (C) 2012 Elsevier B. V. All rights reserved.