An article in the form of a pipe, cable or geomembrane comprising polyethylene or polypropylene, and components A) and B), wherein component A) is a hindered amine light stabilizer containing a triazine residue, component B) is magnesium hydroxide which is present in an amount of 0.01% to 5% by weight relative to the weight of the polyethylene or polypropylene, and the weight ratio of component A) to component B) is 1:50 to 50:1, with the proviso that component B) is not a hydrotalcite.