In a recent PNAS report, we propose that heterolytic activation of a C–H bond in ethylene on a Cr–O bond in silica-supported Cr(III) sites forms a Cr–vinyl species; this surface species coordinates and inserts ethylene to propagate polymer growth (Fig. 1) (1). In their letter, Peters et al. show that the bands at 3,640 cm−1 and 3,605 cm−1 in the infrared spectrum can result from combination bands from the polymer, and not from Si(OH)Cr–R (2). We had already corrected this assignment in a recent publication (3). Using computational data on cluster models presented in ref. 1, Peters et al. describe a kinetic model for the proposed mechanism (2). Their analysis concludes that 1 acts as an “off-cycle trap” (resting state) for most Cr sites, and the C–H bond activation barrier obtained from density functional theory (DFT) studies to form 2 would lead to small amounts of active Si(OH)Cr–R, suggesting alternative initiation pathways. Peters et al. (2) also suggest different chain transfer mechanisms.