Aqueous coumarin was investigated as a possible dosimeter for radiation therapy applications. Coumarin-3-carboxylic acid in aqueous solutions converts upon irradiation to the highly fluorescent 7-hydroxy-coumarin-3-carboxylic acid. The intensity of the fluorescence signal is linearly proportional to the number of the formed 7-hydroxy-coumarin-3-carboxylic acid molecules, which in turn is proportional to the radiation-absorbed dose. Basic characterization of the dosimetric properties (linearity with dose, energy and dose-rate dependence, postirradiation stability, and reproducibility) was performed. The system exhibits nearly linear behavior with dose, in the range of 0.1 to 50 Gy, is stable for at least 166 days following the irradiation, is reproducible within the same solution (+/- 2%) and is energy independent for 6- to 15-MV x-ray energies. A reduction of 18% of the fluorescence signal was observed by changing the dose rate from 0.8 to 4 Gy/min. The origin of the dose-rate effect was investigated and attributed to impurities, predominantly transition metal ions. Removing the impurities by recrystallization of the coumarin and using ultrahigh-purity water reduced the dose-rate dependency to less than 4% in the examined dose-rate range (0.8-4 Gy/min).