Abstrael--.By analytical determination of peroxides of powdered isotactic polypropylene capable of initiating polymerization in an emulsion system at 30 ¢, it was proved that binding of styrene on polypropylene occurs only when the polymerization activator (e.g. chelate FeSO,,-triethylenetetramine) is capable of decomposing those peroxide groups of polypropylene which are more stable than hydroperoxide groups. In other cases, for instance in the presence of triethylenetetramine itself, only homopolymerization occurs, initiated by the hydroperoxide of polypropylene. Hydroperoxide acts negatively on styrene binding on polypropylene as it favours the reaction with chelate FeSO4 triethylenetetramine, the consequence of which is decrease in the amount of the decomposed more stable peroxide initiating the modification reaction. The decomposition of the more stable peroxide of polypropylene in the polymerization system containing chelate FeSO,, triethylenetetramine occurs at 30 ° only up to about 20 rain of the polymerization time. Subsequently polymerization takes place only due to the presence of the long living styrene macroradicals bonded on polypropylene. The dependences of the rate (V,,) of the polypropylene modification by styrene at 30 on the concentration of both the more stable peroxide Ill and chelate FeSO4 triethylenetetramine [CH] are V,.