The size of copper nanoparticles in reverse micelles can be controlled by varying the water content of anionic reverse micelles (AOT = sodium bis(2-ethylhexyl)sulfosuccinate). In the presence of cetyltrim-ethylammonium chloride (CTAC), depending on the concentration, the size of the particles is strongly affected. At a low CTAC concentration ([CTAC] = 4 × 10^-4 M), an unexpectedly large increase in the size is observed. At the other extreme, a decrease in the size is observed at higher CTAC concentration ([CTAC] = 6.4 × 10^-3 M). The increase in the size at low CTAC concentration could be explained in terms of the formation of superaggregates containing most of the cationic surfactant. At higher CTAC concentrations, a random distribution of the positively charged surfactant takes place and the decrease in the size could be due to a decreasein the intermicellar attraction. Furthermore, the yield of particles drastically increases which is attributed to a change in the redox potential.