Investigations were made into the origins and causes of radio interference from high-voltage insulators, and factors which affect the interference level were studied. Pin-type, multicone post-type and pedestal-type insulators were used. In pin-type insulators, over their working voltage range practically all interference was generated in the region of contact between the high-voltage conductor and the porcelain. The interference was reduced to a very low level by applying a conducting paint or a semiconducting glaze to the insulator head. In multicone post-type and pedestal-type insulators most of the interference originated in the metal-cap/first-shed region, and the cement joint between the metal cap and first shed was the predominant source of interference. Control of the voltage stresses by using stress distributor and guard ring, and application of conducting paints to the cement joint, caused a large reduction in the interference and increased the voltage at which interference was first observed. Radio interference from pin-type insulators was also studied in air of different humidities. Increasing humidity caused a decrease in radio interference. The effect was relatively small in the drier conditions, but large reduction in interference was observed in humidity conditions near the saturation value.