3-Hydroxyflavone (3-HF) and 3-deuteroxyflavone (3-DF) were examd. by fluorescence excitation spectroscopy in a supersonic free jet. Compared to 3-HF, vibronic bands of 3-DF are significantly narrower. Substitution of H by D also appears to split vibronic bands into at least three bands leading to a congested spectrum. Fluorescence-dip double-resonance spectroscopy revealed that the complicated spectrum of 3-DF consists of at least three independent partial spectra which are superimposed. The vibrational pattern of every partial spectrum is identical to that of 3-HF, but partial spectra differ in their spectral position. Semiempirical calcns. (MNDO/AM1 with limited CI) were used to generate ground- and singlet excited-state potential energy surfaces as a function of Ph and hydroxyl torsional angles. The deuteration effects suggest the existence of potential energy barriers to low-frequency hydroxyl hydrogen motion, with barrier height on the order of the vibrational zero-point energy.