Nanometric metal clusters in a polymer host were synthesized by a solution-reduction route in a swelling cathode film (SCF) containing metal ions (such as CU2+). The TEM results show that the scale of the clusters is about 10 nanometers and the distribution of their diameter is very narrow. The X-ray diffraction and the electron diffraction patterns confirm that the reduced clusters are the crystallites of metal (copper). The scale of nanometric clusters is affected by the electrochemical conditions in the reactions.