Benzyl (diethoxyphosphoryl)dithioformate (1) has been used to control the AIBN initiated radical polymerization of styrene. In a comparative study with the more popular RAFT transfer agent benzyl dithiobenzoate (2), compound 1 led to higher molecular weight polymers, although with a slightly worse polydispersity. During the polymerization process mediated by 1 a few radicals have been intercepted and characterized by means of ESR spectroscopy. The nature and relative amount of the various radical species varied as the polymerization proceeded, matching expectations based on the generally accepted RAFT mechanism. The present ESR results induce a reassessment of the identity of a radical species observed in a previous preliminary study.