Summary The adsorption of uracil at the mercury electrode-solution interface has been studied by differential capacitance and maximum bubble pressure techniques as a function of pH. At pH 8.0 and below uracil exhibits an initial, dilute adsorption region where the area occupied by one adsorbed uracil molecule, 69±5 2, is close to that expected if the pyrimidine molecule is adsorbed flat on the electrode surface, i. e. with the plane of the ring atoms parallel to the electrode surface. At a bulk solution concentration of about 21 m M and at potentials centered at ca.−0.5 V the uracil molecules undergo an abrupt surface reorientation and adopt a perpendicular stance on the electrode. The area occupied by one uracil molecule in the perpendicular orientation is 39±3 2. At pH 11.0 uracil exists as a monoanion and in this form is significantly less strongly adsorbed in the initial flat surface orientation. The orientation of adsorbed uracil monoanions from the flat to perpendicular stance is not observed at pH 11.0 until the bulk solution concentration of uracil reaches ca. 65m M.