A composition comprising: A. 91.5 to 97.9% of a crosslinkable ethylene-based polymer, e.g., LDPE; B. 1 to 3% of an organic peroxide, e.g., dicumyl peroxide; C. 1 to 5% of a dielectric fluid, e.g., an alkylated naphthalene; and D. 0.1 to 0.5% of a coagent such as AMSD. The compositions exhibit high cure rates without any significant reduction in scorch resistance, heat ageing and electrical performance, and are particularly useful as insulation sheaths for medium and high voltage power cables.