Polyolefin-based plastics insulation contains hydrolysable alkoxysilane(s) as voltage stabiliser, comprising normally liquid cpds. with hetero-atom(s) of formula (R2O)yR' 3-ySi-(R1-R-R' 1-Si(OR2)zR' 2-z)x-OR2 (I); x = 1-10; y = 0-3; z = 0-2 and is variable; R = a gp. of atoms contg. hetero-atom(s); R1 and R' 1 independently = 2-20 C aliphatic or aromatic hydrocarbon gps., opt. with functional substit(s).; R2 = opt. different 1-18C hydrocarbon gps.; R' = a hydrocarbon gp.