All possible monohydroxycoumarin derivatives are modeled using density functional theory computations in order to investigate the role of the hydroxyl group position for the radical scavenging and antioxidant activity of these compounds. Geometry optimization is performed using the B3LYP functional with the 6-311++G(d,p) basis set. The enthalpy changes are assessed in gas phase and in implicit water using the polarized continuum model. Structure-reactivity patterns are delineated. The most reactive isomers as well as the most probable mechanism of interaction between monohydroxycoumarins and free radicals are outlined.