The electronic structure of the derivatives of thiobarbituric acid: 1,3-diethyl-2-thiobarbituric acid ( I) and 1,3-dibutyl-2-thiobarbituric acid ( II) has been investigated by HeI and HeII UV photoelectron spectroscopy ( UPS) and quantum chemical calculations. We discuss their electronic structures and compare them with barbituric acid. We also relate the difference in electronic structure between barbituric and thiobarbituric acids to difference in biological activity of their derivatives. (C) 2010 Elsevier B. V. All rights reserved.