A lubricant composition (A) to improve the melt strength and processability, e.g. extrudability, of a polymer formed from at least one ethylenically unsaturated compound, the composition (A) comprising (1) a heat stabilizer known to be capable of heat stabilizing the said polymer; (2) an external lubricant for the said polymer, and (3) an oxidized polymer, wherein at least 50% of the repeating units of the oxidized polymer are derived from ethylene and any remaining units thereof are derived from at least one alpha -olefin containing from 3 to 10 carbon atoms, and the oxidized polymer exhibits an acid number greater than 5 and a melt viscosity of from 1,000 to about 60,000 centipoises at a temperature of 140 DEG C. … There is also provided a polymer composition (B) comprising a halogen-containing polymer as a major component together with the said lubricant composition (A).