Infrared and UV spectroscopy were used for estimation and mutual comparison of action of Irganox HP 136 and Irganox 1010 in UV stabilisation of polypropylene films. The instantaneous stability state of polypropylene after the different time of UV treatment was also tested by the non-isothermal chemiluminescence so that the actual resistance of sample against the thermal oxidation has been established. It was ascertained that Irganox HP 136 as a weak thermo-oxidation stabiliser is an excellent UV stabiliser of polypropylene, the fact that was not described in the literature until now. This stabilising efficiency was proposed to be provided by photochemical transformation of Irganox HP 136 to 2-hydroxy benzophenone moieties, the compound with ability to absorb the light as well as efficiently scavenge reactive free radicals.