In this paper, the effects of microwave dielectric heating in a multiphase gas–liquid–solid reactor are investigated. Experimental procedures are carried out in order to obtain significant arcing in the bed and control this cold plasma formation interpreted by Maxwell–Wagner effect. These procedures are tested on the catalytic wet air oxidation of phenol using activated carbon as catalyst. This study shows that arcing in the bed can only be obtained under conditions of partial wetting. Arcing leads to hot spots formation and thermal runaway if not early detected, but control is possible by acting on incident power. The low values of phenol conversions obtained are attributed to the very low oxygen partial pressure, experiments having been conducted at atmospheric pressure with air.