With respect to the increasing need for fully characterizing surface-tethered polymer brushes, the capacity of quantitative IR-Fourier transform infrared (FTIR) spectroscopy using a multiple-internal-reflection Si prism as the attenuated total reflection (ATR) element to directly characterize the surface chemical modifications occurring during a surface-initiated controlled polymerization is investigated in the case of high-density polymer brushes. A simple two-step strategy is used involving first the covalent grafting of atom transfer radical polymerization (ATRP) initiators on a hydrogenated silicon surface and the subsequent polymerization of styrene. Three prefunctionalized surfaces designated Si-Br1, Si-Br2, and Si-Br3 are obtained by different procedures. The initiator grafting densities obtained by quantitative IR are 1.7 ± 0.3 nm −2 for Si-Br1, 1.5 ± 0.3 nm −2 for Si-Br2, and 0.9 ± 0.2 nm −2 for Si-Br3. After the polymerization of styrene under the same experimental conditions (grafting from without sacrificial initiators) and a careful Soxhlet rinse to remove physisorbed polymers formed in solution, almost no polymerization is observed using Si-Br1 with a value of the density in polymerized styrene units of 12 ± 2 nm −2 , which is probably due to the chelating effect of the amino linkers used for grafting the initiators in Si-Br1. In contrast, the densities in styrene units are 54 ± 11 nm −2 using Si-Br2 and 141 ± 28 nm −2 using Si-Br3. The degree of polymerization (DP) has been evaluated by measuring the polymer thickness (by ellipsometry and atomic force microscopy, AFM) and using a scaling law relating the latter to DP for dry polymer brushes. High DP values of 200 and 1000 are found in the case of Si-Br2 and Si-Br3, respectively. The fraction of active polymerization initiators is found to be 15−18%, independent of the initiator surface density. In contrast, polymerization kinetics appear affected by steric hindrance and conformational disorder among grafted initiators. This approach for determining surface densities of grafted initiators and grafted polymer chains and DPs is fully generalizable to any other polymer system.