Surface-modified zero-valent copper nanoparticles (CuNPs) are of interest as conductive inks for applications in printed electronics. In this work, we report on the synthesis, stability and characterization of CuNPs formed with a continuous supercritical hydrothermal synthesis method. The precursor, copper formate, was fed as an aqueous solution with polyvinylpyrrolidone (PVP) surface modifier and mixed with an aqueous water and formic acid stream to have reaction conditions of 400 degrees C, 30 MPa and 1.1 s mean residence time. The reaction pathway seemed to proceed step-wise as the hydrolysis of copper formate, followed by dehydration to oxide products and subsequent reduction by hydrogen derived from precursor and formic acid decomposition. The formed surface-modified zero-valent CuNPs had particle sizes of ca. 18 nm, were spherical in shape and contained no oxide contaminants. The formed CuNPs were found to exhibit long-term (>1 year) stability in ethanol as evaluated by shifts in the surface plasmon resonance band of product solutions. Conductive films (0.33 mu m thickness) prepared with the CuNPs had a resistivity of 16 mu Omega cm. The methods reported in this work show promise for producing conductive inks for use in practical printed electronics. (C) 2013 Elsevier B.V. All rights reserved.