An improved electrical insulating oil and oil-filled electrical appliances impregnated therewith. The electrical insulating oil can be produced inexpensively but has excellent electrical characteristics and it comprises a fraction having boiling points in the range of 350.degree. to 450.degree. C. that is prepared by disproportionating diarylalkane or a hydrocarbon mixture mainly containing the same at temperatures in the range of 20.degree. to 500.degree. C. in the presence of a disproportionation catalyst, said diarylalkane having boiling points in the range of 260.degree. to 320.degree. C.