Epoxy thermosets have revolutionized the coating, adhesive, and composite industries but the chemicals from which they are synthesized have significant effects on the environment and human health not only precure but also after crosslinking has occurred. In this study, we propose tannic acid (TA) as an alternative epoxy hardening agent for commercially available epoxy resin, the diglycidyl ether of bisphenol A (DGEBA). The resulting thermosets were characterized by Fourier transform infrared spectroscopy, optical microscopy, dynamic mechanical analysis, differential scanning calorimetry, compression testing, and thermogravimetric analysis. The results from this study showed that at temperatures above 100 degrees C, the compatibility of TA in DGEBA was significantly increased for loading levels up to 37% weight of TA in DGEBA, something that has not been seen before in literature. It was also discovered that at high loading levels, the resulting materials had glass transition temperatures at and above 200 degrees C. The resulting material was proposed as a more sustainable alternative to amine or acid hardened epoxy thermosets and was particularly useful in high-temperature applications. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1468-1480