ABSTRACTHydrolyzed proteins previously shown to aggregate in aqueous solution were compounded into synthetic polyisoprene rubber (IR). Modulus increases of up to 232% resulted from protein reinforcement of IR. Increased hydrogen bonding on amine groups and the presence of β‐sheets in the protein phase were observed via Fourier transform infrared (FTIR) spectroscopy. The total β‐sheet amount relative to the IR content strongly correlated to the modulus and varied with the protein concentration, protein aggregation state, and compounding conditions. Isotropic protein aggregates on the order of hundreds of nanometers were observed by scanning electron microscopy with energy dispersive x‐ray spectroscopy (SEM‐EDX). The aggregates were evenly dispersed throughout the rubber matrix after compounding. The composite glass transition temperature (Tg) was unchanged from the control, which indicated that the protein and rubber existed as two discrete phases. Remarkably, protein β‐sheet structures were observed in FTIR even after rubber compounding under harsh conditions. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2018,135, 46026.