Emission is observed at the surface of a Pt working electrode with a small surface area of 1 mm(2) at an applied voltage of 400 V or more, under cathodic polarization, as well as at the voltage of 900 V or more, under anodic polarization. An emission spectrum observed at the cathode in 0.5 M NH4NO3 solution consists of H, O, Pt, and H-2 lines which are originated from H2O and the electrode. The ratio of the intensity of O line at 777.1 nm to H line at 486.1 nm is large at the anode, in comparison with that observed at the cathode. A strong emission from OH radicals is observed in acidic solutions of HNO3, H2SO4, and HCl, and in basic solutions of KOH and NaOH, except for neutral solutions containing NO3-ions. The H-2 and Pt emissions are quenched by Kt ions and are fairly weakened by Na+ ions. Nevertheless, no quenching of the Pt and H-2 emissions is found in solutions containing Li+ ions. These results suggest that emission spectra are affected by electrochemical reactions, e.g., gas evolution and electroplating at the electrode surface. Furthermore, it is elucidated that the generation of hot plasma at the cathode is due to not only joule heating of the electrolyte but also the injection of electrons with high kinetic energy of a few hundred electronvolts.