<p>Due to the presence of several components in epoxy composites, monomers, crosslinking agents, fillers, reinforcements and other additives, the final properties and performance of the product depend strongly on their physicochemical properties and ratios of these components. In this work, the influence of natural polyphenolic compounds, tannins as bio-based additive on various properties of commercial epoxy resin was investigated. Tannins were isolated from the tree’s bark obtained as a byproduct from wood industries, purified, and they were utilized as sustainable fillers in epoxy composites. Different ratios of tannins (0, 5, 20 and 40 wt%) were added to the epoxy resin and hardener mixture. The curing behaviour, thermal properties and curing kinetics of tannin/epoxy composites were investigated. Also, the potential of tannins to utilization as crosslinking agent into epoxy resin were characterized by means of DSC. The results showed that the epoxy resin loaded with tannins reached 100% curing percentage and exhibited the same curing behaviour as pure epoxy resin, which is characterized by a single exothermic peak observed in the temperature range of 90–150&#xa0;°C with a peak temperature of 130–140°C. An&#xa0;increase of 4°C was found&#xa0;in the glass transition temperature (<i>T</i>g) in higher tannin content (40 wt%) composite. The addition of tannins without hardener has no influence on the curing process of pure epoxy. The values of activation energy (<i>E</i>a) were in the range of 45–56&#xa0;kJ&#xa0;mol^–1 and considered within the accepted ranges of most commercial epoxy composites. The results also showed that the tannins offer competitive advantage properties and have been considered one of the most promising materials as renewable fillers in epoxy-based composites.</p> Graphical abstract <p></p>