This paper describes recent efforts to characterize the diffusion of single molecule SERS analytes adsorbed to the surface of aggregated silver nanoparticles. Using a two-dimensional Gaussian function to fit the point spread function of a single diffraction-limited SERS emission image, the centroid of the emission can be located with precision better than 5nm. Tracking the centroid over time allows a diffusion trajectory to be constructed. By calculating the mean-r-squared displacement as a function of time lag, the diffusion properties of the SERS centroid can be tracked. For the single molecule SERS centroids presented herein, the diffusion analysis shows signatures of subdiffusion, indicating barriers to Brownian diffusion on the nanoparticle surface.