An improved method for imparting excellent long-term dielectric performance to an in-service electrical cable section having a stranded conductor surrounded by a conductor shield encased in a polymeric insulation and having an interstitial void volume in the region of the conductor, the cable section having an average operating temperature T. The method comprising injecting a dielectric enhancement fluid composition into the interstitial void volume, the composition comprising at least one component selected from: (1) a water-reactive material selected from an organosilane monomer, the above organosilane monomer wherein at least one of the water-reactive groups has been substituted with a condensable silanol group, an oligomer oligomer of the above organosilane monomer, or a co-oligomer of the above organosilane monomer, the organosilane monomer having a diffusion coefficient at least about 15 times greater than the diffusion coefficient of its corresponding tetramer; (2) a water-reactive material similar to (1) having at least one group attached to silicon comprising 7 to about 20 --CH.sub.2-- units; (3) a non-water-reactive organic material which has a diffusion coefficient of less than about 10.sup.-9 cm.sup.2/sec and an equilibrium concentration of at least about 0.005 gm/cm.sup.3 in the polymeric insulation, the above mentioned diffusion coefficients and equilibrium concentration being determined at temperature T; or (4) an organic compound having an equilibrium concentration in the polymeric insulation at 55.degree. C. which is less than 2.25 times the equilibrium concentration at 22.degree. C.