The microviscosity of aqueous micelles of 60 surfactants differing by the nature of the headgroup (cationic, anionic, nonionic, and zwitterionic), the carbon number of the surfactant alkyl chain, the nature of the counterion, and the chemical structure of the surfactant (conventional, bolaform, dimeric) has been investigated by fluorescence probing, using the viscosity-sensitive fluorescence emission of the probe 1,3-dipyrenylpropane (P3P). The microviscosity was taken as the product Q of the P3P excimer lifetime tau(E) by the ratio I-M/I-E Of the emission intensities of the P3P monomer and excimer forms, respectively. The energy of activation E* of the microviscosity was obtained from the variation of Q with temperature in the range between 15 and 45 degrees C. The results for series of homologous surfactants showed that both Q and E* increase with the alkyl chain length. The increment of E* per additional methylene group in the;alkyl chain was found to be independent of the nature of the surfactant series investigated and close to the increment of E* for n-alkanes, for alkyl chains containing 8 --> 12 carbon atoms, within the experimental error. This led to the conclusion that the large difference that exists between the values of Q for an n-alkane and the micelle of a surfactant having the same alkyl chain length is essentially due to the tethering of the surfactant alkyl chains to the micelle surface Q varied in an irregular fashion with the size of the surfactant headgroup, but E* was nearly independent of this parameter. In the case of the nonionic ethoxylated surfactant micelles, which possess a thick shell made of the polar heads, this result reveals that the probe is predominantly located in the micelle core, close to the micelle surface. The values of Q and E* were found to be significantly lower for anionic surfactant micelles than for micelles of cationic, nonionic, and zwitterionic surfactants. The differences were attributed to a location of P3P somewhat closer to the surface of the micelle core of the latter surfactants; owing to the existence of attractive interactions between the probe and the surfactant polar heads. The results concerning the effects of the surfactant chain length, size of the polar head, and counterion nature showed the importance of the packing of the surfactant alkyl chains in determining the micelle microviscosity. Last, the microviscosity of micelles of cationic surfactants having the same chain length and headgroup was found to vary with the surfactant structure following the sequence: conventional surfactant < bolaform surfactant < dimeric surfactant.