Materials used in the research are low density polyethylene (LDPE) pure and in the composites with glycerin-modified starch (S) of various content. The aim of the present research was to specify the effect of UV radiation on a stability of the LDPE/S composite by analyzing FTIR spectra and discussing kinetics of photolysis. It was found that the modified starch protected LDPE in the LDPE/S composites against UV radiation. A decrease of rate constant k of LDPE photodegradation was reported. Results of water adsorption measurements of the composites are presented in the form of water adsorption kinetic curves and adsorption isotherms which were described well by the GAB model.