We investigated the photoelectrochemical properties of both n- and p-type (In,Ga)N nanowires (NWs) for water splitting by in situ electrochemical mass spectroscopy (EMS). All NWs were prepared by plasma-assisted molecular beam epitaxy. Under illumination, the n-(In,Ga)N NWs exhibited an anodic photocurrent, however, no O-2 but only N-2 evolution was detected by EMS, indicating that the photocurrent was related to photocorrosion rather than water oxidation. In contrast, the p-(In,Ga)N NWs showed a cathodic photocurrent under illumination which was correlated with the evolution of H-2. After photodeposition of Pt on such NWs, the photocurrent density was significantly enhanced to 5 mA/cm(2) at a potential of -0.5 V/NHE under visible light irradiation of similar to 40 mW/cm(2). Also, incident photon-to-current conversion efficiencies of around 40% were obtained at -0.45 V/NHE across the entire visible spectral region. The stability of the NW photocathodes for at least 60 min was verified by EMS. These results suggest that p-(In,Ga)N NWs are a promising basis for solar hydrogen production.