Porous Si was formed on n-Si wafers, modified with fine Pt particles, by simply immersing the wafers in a HF solution without a bias or an oxidizing agent. The Pt particles were deposited onto n-Si wafers by electrodeposition or electroless displacement deposition. SEM images show that many pores, ranging between 0.1 and 0.8 μm in diameter and covered with a luminescent nanoporous layer, were formed only on the Pt-modified area of the n-Si surface by immersion in 7.3 M HF solution for 24 h. The weight loss of Pt-electrodeposited n-Si wafer was 0.46 mg cm -2 , corresponding to ca. 2 μm in thickness. The weight loss and the structure of porous Si changed with the etching conditions, such as concentration of dissolved oxygen in the HF solution, distribution density of metal particles, and different kinds of metal particles. A photoelectrochemical solar cell equipped with a Pt-particle-modified porous n-Si electrode gave 13.3 mW cm -2 of maximum output power, which corresponds to a 13% conversion efficiency and is higher than that for the Pt-particle-modified flat n-Si electrode. ©2003 Elsevier Science B.V. All rights reserved.