Phosphines have been successfully used as additives for Fe-based ATRP. To understand their role in detail, the kinetics of Fe-based ATRP in the presence of tris(2,4,6-trimethoxyphenyl)-phosphine (TTMPP) and of triphenylphosphine (TPP) was studied via online vis/near-IR and P-31 NMR spectroscopy. No significant effect on the ATRP equilibrium constant, K-ATRP, was observed upon the addition of up to 1.5 equiv of phosphine relative to Fe. The strongly Lewis basic TTMPP may coordinate to Fe-II species, but primarily it acts as a reducing agent for [(FeBr4)-Br-III](-) in ATRP above 100 degrees C, thus transforming TTMPP to TTMPP-Br+. TPP is a weaker Lewis base and consequently a less effective reducing agent.