The deposition of metallic nickel on n-Si(100) wafers was performed without external potential control in aqueous NiSO4 solutions of different compositions at pH 8.0. Without giving any catalyzation treatment, the deposition of nickel on hydrogen-terminated Si(100) was confirmed in a conventional electroless plating bath containing NaH2PO2 as the reducing agent, sodium citrate as the complexing agent, and (NH4)(2)SO4 as the buffering agent. The deposition of nickel was found to take place also in a bath without the reducing agent, and even in a simple solution consisting of NiSO4 and (NH4)(2)SO4. By using a transmission electron microscope equipped with an energy dispersive X-ray spectrometer, the cross sections of the films deposited from these solutions were examined, which revealed formation of silicon oxide between the Ni deposit and Si substrate. Based on these results, the mechanism of the entire process of electroless Ni deposition on Si is discussed.