New reactive surfactants (RS) and synergistic reactive surfactants (SRS), containing reactive groups and longer unsaturated apolar hydrocarbon chains have been synthesised and applied in filled and flame-retarded polymer systems, respectively. These additives are capable of reactions with the polymer matrix and the disperse phase as well. The reaction of these compounds with polyolefins was proven on model system by surface analysis, while their reaction with fillers was demonstrated by DSC. The interface modification in filled/reinforced polymer systems and polymer blends resulted in improved mechanical properties. while in pigmented PE it contributed to higher photostability. Furthermore, the efficiency of intumescent flame retardant additives in polyolefins could be enhanced.