Abstract: Process for the long-duration removal of polychlorinated biphenyls (PCB) from transformer-insulating liquids when substituting insulating liquids, particularly silicon oils, for PCB oils. The PCB contaminants can be removed continuously or substantially continuously from the transformer-insulating liquids using an adsorption resin, thus preventing PCB from accumulating in the transformer-insulating liquid. The insulating liquid is passed substantially continuously through the adsorption resin; adsorption resin, after enrichment with PCB oil, is washed with a solvent for PCB oils; and the washed resin is freed from residual solvent by rinsing it with a gas prior to reusing the thus regenerated resin to adsorb additional PCB oil.