Gamma radiation is an effective method for microbial decontamination of plant materials. However, this treatment can induce changes in the chemical structure of plant constituents. Stem barks of Luehea ochrophylla were exposed to different doses of gamma radiation to evaluate decontamination efficiency and changes in chemical composition of plant material including phenolic fraction. The major microbial contaminants of non-irradiated samples were isolated and identified as the fungal species Eurotium chevalieri L. Mangin and Lecythophora decumbens. The dose of 5.0 kGy was effective to achieve total decontamination of the stem barks of L. ochrophylla. The formation of free radicals was verified in the plant material using electron paramagnetic resonance spectroscopy, and was supposed to be related to the trans-aconitic acid, a plant constituent. It was the only secondary metabolite to have its concentration significantly altered with radiation in phenolic fraction, as observed by liquid chromatography with diode array detector coupled to mass spectroscopy (LC-DAD-MS). The trans-aconitic acid was isolated and exposed to gamma radiation in aqueous medium. Its concentration decreased after exposition to a dose of 3.0 kGy, corroborating the supposition of its degradation. Citric acid was the main radiolytic product formed by irradiation of trans-aconitic acid in the presence of water.