The work is devoted to the study of the characteristics of a low-temperature plasma of a high-frequency (HF) discharge (f = 13.56 MHz) ignited between metallic and electrolytic electrodes at atmospheric pressure. Burning of an RF discharge in a diffuse (volumetric) form is observed at the interface between the media between the electrodes. Numerical calculations of the electric field strength and the distribution of the volumetric power density of the Joule heat release before breakdown in a vapor-gas mixture near a metal electrode are presented. The discharge radiation spectrum, plasma composition, and electron density were studied by optical emission spectroscopy. The thermograms of the electrode surface under the conditions of an RF discharge are considered.