A positive-microemulsion system (water/sodium linoleate/hexane) was used to make uniform Ag2Se nanoparticles at low temperature (7-10 degrees C) within five to ten minutes. The proposed interface-reaction mechanism was corroborated by transmission electron microscopy (TEM), IR spectroscopy, and X-ray photoelectron spectroscopy (XPS) results. This is a general method that is ideal for making other uniform nanoparticles on a large scale through simple precipitation reactions.