Detailed investigations of the spectral diffusion of sulforhodamine 101 (SR101) in polymer films have been performed over a range of concentrations from typical concentrations for single molecule (SM) spectroscopic studies (10(-10) M) to typical concentrations for bulk (10(-6) M) experiments. A ''single-molecule'' ensemble spectrum, obtained by averaging 42 different SM spectra of SR101 in a poly(methyl methacrylate) film, exhibits extreme broadening and a large vibronic peak that is not observed in the more concentrated samples. The distribution of SM spectral means is extremely bread (FWHM= 616 cm(-1)) with molecule-to-molecule variations that last at least several hours. Autocorrelation analysis of 170 SM spectral mean trajectories on the second times scale reveals that the most probable behavior for SR101 molecules is to exhibit negligibly small amounts of spectral diffusion. The observed effects, i.e., the large vibronic peak, the broad spectral distribution, and rare rapid jumps in the spectral mean, are assigned to differences in probe nanoenvironments. The effects of association with these ''rare'' sites are saturated at extremely low (10(-9) M) concentrations. The unexpected concentration dependence observed here could have implications for data interpretation in many SM experiments.