Background: A decoction prepared with barks of Adenanthera pavonina and Thespesia populnea is a herbal formulation which has been prescribed in Sri Lanka in the treatment of cancer patients for many years. This study was designed to investigate its phytochemical and antioxidant properties. Materials and Methods: The total phenolics and flavonoids were determined using Folin-Ciocalteau and aluminum chloride colorimetric methods, respectively. Gallic acid content in the decoction was determined using high-performance liquid chromatography. The antioxidant properties were evaluated by 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical assay, nitric oxide scavenging assay, deoxyribose method, and the reducing power of the decoction. Results: The concentration of total phenols, flavonoids, and gallic acid of the decoction were 34.13 ± 3.54 w/w % gallic acid equivalents, 41.37 ± 0.57 w/w % of (-)-Epigallocatechin gallate equivalents, and 0.58 ± 0.24 mg/g, respectively. The EC50 for DPPH, nitric oxide scavenging, and deoxyribose assays were 7.24 ± 0.50, 14.02 ± 0.66, and 53.21 ± 2.82 μg/ml, respectively. Reducing power of the decoction increased with the concentration. Conclusion: These investigations suggested that the decoction prepared with A. pavonina and T. populnea can be a potential source of nutraceuticals with antioxidant activity.