An improvement is provided in the forming and vulcanizing of organic peroxide vulcanizable polymers in a long-land die by the use of a special class of forming coagents characterized byA. a viscosity of at least 0.5 centi-stokes and at most 3,000 centi-stokes at 235.degree.C.,b. an absorption ratio to the said polymer at 150.degree.C. for 45 hours less than 100 mg./cm.sup.2.,C. not being gelled even in contact with said organic peroxide in the course of vulcanization, andD. not being boiled in the course of vulcanization.Electrical cable having a vulcanized insulation layer made according to the present invention is characterized by an excellent continuous surface layer free of voids and bubbles.