Viscosities of aqueous micellar solutions of cetyltrimethylammonium bromide (CTAB) and Triton X-100 were measured in the presence and absence of coumarin derivatives, mainly 6-alkyl coumarins with alkyl substituents between methyl and cetyl. The coumarins generally increased the viscosity; the extent of the increase was specific: alkyl chains of medium length showed the largest effects. In situ photodimerization of the coumarins led to a further rise in the viscosity for short alkyl substituents but to a drop in the viscosity in CTAB for long alkyl substituents. Thermodynamic parameters of micelle formation, determined from surface tension measurements, correlate with rheological changes and indicate an increased hydration of the micelles in the cases exhibiting a high viscosity.