The durabilities of polyethylene and polypropylene, in the presence and absence of added titanium dioxide, towards photodegradation have been assessed by the rate of oxygen absorption upon irradiation, measurement of photo-induced carbonyl compound and hydroperoxide formation using IR spectroscopy, and by measuring the change in tensile propertics caused by irradiation. Measurement of carbonyl compound and hydroperoxide formation by IR spectroscopy is suitable for polyethylene but not for polypropylene. The most sensitive method of following degradation appears to be the measuring of change in tensile properties. In general, the lower the photo-reactivity of added titanium dioxide, the greater its protecting action provided that the pigment is used at sufficiently high loading. The manner in which a pigment is dispersed in a polymer affects the performance of the pigment as a stabilizer. -- AATA