Benzyl (diethoxyphosphoryl)dithioformate (1) and benzyl (diethoxythiophosphoryl)dithioformate (2) were studied as RAFT agents in the polymerization of styrene. The reactive intermediates involved in the process were identified by electron spin resonance spectroscopy as the species deriving from the addition of the polymer propagating radical to the chain transfer agent, in agreement with the RAFT mechanism. Both thermally and AIBN-initiated RAFT styrene polymerizations were performed at different temperatures. In general, the molar mass increased with time as expected for a controlled polymerization process. The molar mass distribution also progressively increased. Inefficient control of the molar mass distribution appears to be related to the rate of radical formation, which is not sufficiently fast compared to the overall monomer conversion, as well as to the rate of bond dissociation and reformation with respect to the propagation rate. As a result, both 1 and 2 behave as RAFT agents but, at relatively high conversion, afford polymers endowed with polydispersities as large as in conventional radical polymerizations.