Forward recoil spectrometry (FRES) has been used to monitor the extent of reaction between model end functional polymers at a PS/PMMA interface. Bilayer samples comprising a high molecular weight polystyrene layer containing approx. 8.5 wt% of amino-terminal deuterated polystyrene (dPS-NH2) on an anhydride-terminal poly(methyl methacrylate) (PMMA-anh) layer were annealed at 174°C for various periods. Significant reaction with the PMMA-anh, indicated by an interfacial excess, was observed with three molecular weights (M = 22 000, 37 000, and 92 000 g/mol) of dPS-NH2. The measured growth of the interfacial excess with time for all three molecular weights could not be well described with recent theories of diffusion-controlled interfacial reactions. A model that incorporates polymer chain reactivity fit the data better, with reaction rate constants k = 0.20, 0.055, and 0.042 kg mol-1 s-1 for dPS-NH2-22, -37, and -92, respectively. Furthermore, a novel trilayer sample geometry showed that the reactive dPS chains can diffuse throughout the PS matrix and sample the PS/PMMA interface several times before a significant amount of reaction occurs. The results of this study are consistent with other experimental systems where the in situ copolymer formation is limited by reaction rate, rather than the diffusion of reactive chains through the bulk.