The small-signal charge transfer inefficiency (SCTI) of a surface-channel CCD has been studied. The experimentally observed behavior of the SCTI could not be explained by the conventional interface state model. Using the McWhorter model for the interface states, which assumes a distribution of the states in the oxide perpendicular to the interface, an excellent agreement between theory and experiment is obtained. Essentially, this is due to the fact that in the McWhorter model a spectrum of capture cross sections is associated with each energy level. No evidence for the edge effect has been found in devices with a channel width above 50 µm.