Atom-transfer radical polymerization (ATRP) of styrene catalyzed by chloro-substituted amine-bis(phenolate)iron complexes was investigated via online vis/NIR spectroscopy up to 6000 bar. Pressure significantly increases the rate of ATRP by enhancing both the equilibrium constant, K-ATRP, and the propagation rate coefficient, k(p). The observed negative reaction volume, Delta V-r(K-ATRP), is due to a stronger contraction of the ligand sphere for the Lewis-acidic Fe-III complex compared to the Fe-II complex. Despite the enormous rate enhancement of ATRP, narrow molar mass distributions may be obtained even at very high pressure and thus very high rates.