The structures of sooting and polycyclic aromatic hydrocarbon (PAH) zones in counterflow diffusion flames have been investigated experimentally. The rate of soot mass growth is modeled, considering C2H2 addition to the surface of soot particles and coagulation of PAHs with soot particles. The results show that hydrocarbon radicals can play an important rate in the activation of a soot particle's surface for C2H2 to add to it when free H atoms are nearly absent from the soot growth region. Coagulation of PAHs with soot is also important far soot growth in the low temperature region of diffusion flames, which have relatively high concentrations of PAHs. The combined mechanism of C2H2 addition to soot and PAH-soot coagulation is capable of successfully predicting the growth rate of soot particles determined experimentally.