Lateral dispersion over the continental shelf was examined using dye studies performed as a part of the Coastal Mixing and Optics experiment. Four experiments performed at intermediate depths, each lasting 2.5--5 days, were examined. In some cases the dye patches remained fairly homogeneous both vertically and horizontally throughout an experiment. In other cases, significant patchiness was observed on scales ranging from 2 to 10 m vertically and a few hundred meters to a few kilometers horizontally. The observations showed that the dye distributions were significantly influenced by shearing and straining on scales of 5--10 m in the vertical and 1--10 km in the horizontal. Superimposed on these larger-scale distortions were simultaneous increases in the horizontal second moments of the dye patches, with corresponding horizontal diffusivities based on a Fickian diffusion model of 0.3--4.9 m2 s-1. Analysis of the dye data in concert with shear estimates from shipboard acoustic Doppler current profiler observations showed that the existing paradigms of shear dispersion and dispersion by interleaving water masses cannot account for the observed diffusive spreading of the dye patches. This result suggests that some other mechanisms provided an additional diffusivity of order 0.2--4.6 m2 s-1. An alternative mechanism, dispersion by vortical motions caused by the relaxation of diapycnal mixing events, may explain the observed dispersion in some cases. ¿ 2001 American Geophysical Union