The adsorption and reorientation of quinoline has been studied by measuring differential capacity. In addition to the ordinary differential capacity-potential curves, the differential capacity-time curves by a flow injection method were measured. It was observed in the differential capacity-potential curves that reorientation peaks between two adsorption potential regions exist. By the flow injection method it was found that the adsorption of quinoline on a mercury electrode is reversible at any potential. This means that thermodynamic treatments of electrocapillary curves have meaning. Reorientation of quinoline is also observed in the differential capacity-time curves. It is tentatively considered that the adsorption of quinoline is not by the replacement of water molecules at the primary water layer of the double layer, but by the replacement of water molecules not adjacent to the electrode or hydrated cations.