The lipid/surfactant mixed interactions between the lipid dipalmitoylphosphatydilcholine (DPPC) and the nonionic surfactant C12E7 (C12H25(OCH2CH2)(7)OH) were studied by the use of fluorescence anisotropy of nile red and DCM laser dye. Three different regions consisting of mixed micelles, mixed vesicles, and both aggregates were identified. Nile red fluorescence anisotropy is wavelength-dependent, the spectra being decomposed into two log-normal functions. These results are consistent with a solvent relaxation process, with distinct anisotropies for the relaxed and unrelaxed states. The spectral properties (anisotropy and maximum emission wavelength) of the relaxed state show higher sensitivity to the environment than those of the unrelaxed state. DCM fluorescence anisotropy shows similar trends. This was interpreted as a result of a trans-cis photoisomerization process. The observed anisotropy spectrum was decomposed in two Gaussian functions reflecting a distinct anisotropy for each isomer.