A novel experimental setup to measure the diffusion and the concentration of light-absorbing molecules in particles of polymer dispersions is described. This method allows for instance quantification of the diffusion of hydrophobic reversible addition-fragmentation chain transfer (RAFT) agents from the monomer phase into the polymer particles during ab initio emulsion polymerization. The results clearly show that the diffusion of the RAFT agents strongly depends on their water solubility. This explains their influence on the kinetics of RAFT aqueous heterophase polymerization. Moreover, the retaining or the loss of the color of the RAFT agents after addition of different initiators recorded by this method gives direct experimental evidence that primary initiator radicals enter polymer particles.