A new mathematical model of Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization is proposed, describing a series of experimental data on styrene bulk polymerization in the presence of various RAFT agents. The polymerization slowdown observed with increasing concentration of the RAFT agent is attributable to the accumulation of intermediate products that participate in chain transfers to both low-molecular and polymer RAFT agents, whereas the length of the initiation period is determined entirely by the activity of the low-molecular RAFT agent.