An on-line time-resolved fluorescence technique is applied to study local dynamics in polymer particles during an emulsifier-free heterogeneous polymerization. The polymerization of styrene in the aqueous phase, initiated with a polymeric initiator (hydrolyzed propene-maleic acid copolymer with tertbutyl perester groups) was chosen as an illustrative system. To analyze the heterogeneous reaction pyrenyl probes were attached to the backbone of the initiator. Within the end phase of the polymerization an increase of fluorescence lifetime and intensity was observed. Fluorescence quenching experiments of the photo excited pyrene with dibenzoyl peroxide in toluene proved that the perester groups act as quencher. The calculated quenching constant points out that the quenching reaction is diffusion controlled. The also investigated bulk polymerization of styrene with dibenzoyl peroxide as initiator and pyrene as fluorophore showed that a change of the quenching probability at the gel point is responsible for a fluorescence lifetime and intensity enhancement. The emulsion polymerization showed the gel effect, too, although the macro-initiator forms a shell around the polystyrene core. At the gel point the chain mobility is reduced so that the encounter probability of pyrenyl and perester groups is limited. Despite the turbidity of the reaction mixture, fluorescence techniques allow to study the dynamics in the polymerizing particles.