A new method to control the size and distribution of electrodeposited metal on a semiconductor was investigated, using a system of Pt deposition on p-type Si. Pt is a noble metal, and electrodeposition is possible on p-type Si through hole injection to the valence band. When the Si surface is illuminated, an additional charge-transfer path becomes possible utilizing electrons photo-excited to the conduction band. The two pathways give different morphologies for the deposits. Fine and dispersed particles were electrodeposited under illumination, indicating the nucleation process was prevailing; in the dark, grown deposits were observed and the dominant process was crystal growth. We used the different deposition behavior to control the size and distribution of electrodeposits by the illumination-modulated method. Initial illumination gives nuclei of electrodeposits, and the adjustment of the duration can control the number of electrodeposits. The following electrolysis in the dark grows the deposits and can control the size. The method showed the possibility of controlling the size and distribution of Pt electrodeposits on p-type Si without changing applied potential. (c) 2005 The Electrochemical Society.