AbstractDibasic lead phosphite is known to be a stabiliser for plasticised PVC against weathering. The stabilisation mechanism, however, is not known and could be due to absorption of u.v. radiation by the dibasic lead phosphite, or to an antioxidant action.It is suggested that the mechanism, in fact, depends upon an antioxidant action against the harmful effect of atmospheric oxygen absorbed during the test, and also that a similar effect occurs during thermal degradation of the PVC in free air. This beneficial effect is due to the presence of the phosphite grouping, but it is also necessary to have sufficient basic lead present to remove chlorine ions or radicals in the form of lead chloride. It has been found that equivalent molecular amounts of dibasic lead phosphite and normal lead phosphite are equally efficient in the presence of basic lead sulphate compounds.An interesting discovery is that the weather conditions during the early stages of an exposure test may be more important than the total radiation received by the test piece at a later date. The conditions in the early stages of a test influence the manner in which the basic lead stabiliser removes the harmful chlorine ions or radicals liberated from the polymer. Mild conditions allow sufficient time for lead to diffuse through the plastic to the surface region where it reacts to form a precipitate of lead chloride in the form of needle like crystals. Harsher conditions result in the basic lead stabiliser particles themselves absorbing the chlorine ions or radicals to form lead chlorine in situ.Tetrabasic lead sulphate improves the stability of the PVC by absorbing some of the harmful u.v. radiation with the result that the diffusion mechanism is favoured even under severe conditions.