The chain cleavage mechanisms during hydrogen peroxide catalysed photo-oxidative degradation of high molecular weight natural rubber, which was described as producing liquid natural rubber is elucidated. NR is photolysed in toluene solution(2.5 wt.;) in a photoreactor at 30 oC under atmospheric air and nitrogen, separately, with a 400 W medium pressure mercury vapour lamp( =>_290nm) with constant stirring in the presence of hydrogen peroxide and/or titanium dioxide. The decrease in viscosity-average molecular weight(Mv) was determined by viscosity measurements. The photoproducts were characterised by FT-IR, 1H and 13C NMR spectroscopies. Burfields and the present results gave indication for chain cleavages resulting from the classical free-radical oxidation process rather than a direct reaction of hydroxy radical at the methylene-methylene bond. The hydrogen peroxide and/or allylic hydroperoxides acts as photoinitiator by producing hydroxy radicals and/or alkoxy radicals. Moreover, the concomitant addition of 1 O2 (singlet oxygen) via the well known ene reaction producing hydroperoxides, can not be neglected.