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trans-Resveratrol was evaluated in a stability study. Reverse-phase high-performance liquid chromatography with a diode array detector was used for the comparison of stressed and reference samples. Aqueous ethanolic solutions were examined under variable conditions. The following parameters were investigated: the time of storage, exposure to sunlight for up to 30 days, temperature from 5 to 80°C, pH from 2.9 to 10.2, trans-resveratrol concentrations from 0.5 to 100 mg L−1, and 3, 10, 20, and 50% ethanol. The cis/trans equilibrium position was significantly influenced by the resveratrol concentration. The trans-resveratrol isomer was stable only at solutions more concentrated than 25 mg L−1that were stable for 30 days in a refrigerator or at room temperature protected from light. Degradation of no more than 10% was observed at temperatures lower than 50°C and pH values lower 7.43. [ABSTRACT FROM PUBLISHER]