Abstract: An insulating composition comprises a polyolefin, preferably polypropylene, and hollow microspheres, at least some of which have been pre-treated with a chain-scission agent and, optionally, a functionalizing agent for the polyolefin, before addition to the polyolefin melt. The provision of the chain-scission on the microspheres enables the microspheres to be compounded substantially without breakage with low Melt Flow Index polyolefins, with the result that the resultant composite material can be employed in rigorous application areas, for examples as an extruded flowline insulation.