The addition rate constants of five differently substituted benzoyl radicals to butyl acrylate were determined by continuous-wave time-resolved EPR (TR-EPR at X-band (9 GHz) and at W-band (95 GHz)) spectroscopy. Remarkably, the reactivity of the benzoyl radicals, is divided into two domains: One is established at low (< 1.25 M) concentrations of butyl acrylate with rate constants in the range of 106 mol(-1) s(-1). At alkene concentrations above ca. 1.25 M, the addition reaction slows down by -1 order of magnitude. This decrease of the reactivity coincides with a dramatic increase of the macroscopic viscosity in the reaction medium.