New μ-hydroxido/oxido bridged trinuclear uranyl(VI) complexes with 3,5-di-t-butyl-N-salicylidene-2-aminophenolato (dbusap) ligands, EtNH[(UO)(μ-OH)(dbusap)] (EtNH[1]) and (EtNH)[(UO)(μ-O)(dbusap)] ((EtNH)[2]) were synthesized and characterized. Single-crystal X-ray structures of both complexes were determined. The oxygen atom on μ-hydroxido center in [1] is sp hybridized with an average U-(μ-O)-U bond angle of 109.7(5)°; the μ-oxido atom in [2] is sp hybridized with an average U-(μ-O)-U bond angle of 118.0(10)°. U-(μ-O) distances in [1] are long (average of 2.43(1) Å) compared with those in [2] (average of 2.23(2) Å). The optimized geometries of the [(UO)(μ-OH)] core in [(UO)(μ-OH)(sap)] and the [(UO)(μ-O)] core in [(UO)(μ-O)(sap)] (where sap = N-salicylidene-2-aminophenolato) from density functional theory (DFT) calculations resemble those in [1] and [2], respectively. The U-(μ-O) bond in [2] is significantly shorter than that in [1], because of the greater negative charge on the central μ-oxido. A reversible structural conversion between [2] and [1] was conducted by protonation and deprotonation of the μ-oxido/hydroxido group. The activation enthalpy and entropy of the proton self-exchange reaction between [1] and [2] determined from the temperature dependence of H NMR coalescence are ΔH = 23 ± 2 kJ mol and ΔS = -77 ± 5 J K mol.