The effects of addition of the photosensitizer riboflavin (RF) to semidilute solutions of the systems ethyl(hydroxyethyl)cellulose (EHEC)/water, EHEC/sodium dodecyl sulfate (SDS), and EHEC/cetyltrimethylammonium bromide (CTAB) on the turbidity and the linear viscoelasticity are studied. The turbidity behavior and the cloud point (CP) are influenced by the addition of RF to the EHEC/SDS system, whereas no discernible change is observed for the other systems. The rheological features of all systems are affected by the presence of RF at lower temperatures, whereas at temperatures close to the CP, only a slight effect is detected. Both the EHEC/SDS and EHEC/CTAB systems evolve thermoreversible gels at the same temperature (37.5degreesC), but in the presence of RF, the EHEC/CTAB system does not form a gel, whereas the gel temperature for the EHEC/SDS system is depressed (32.5degreesC). Light irradiation of RF in the EHEC/SDS/RF system causes fragmentation of the network and a higher temperature is required to re-form the incipient gel network. The photochemical degradation of EHEC gives rise to a decrease in the dynamic moduli and the complex viscosity for all of the three systems. The effect is strengthened at higher temperatures and it is most pronounced for the EHEC/SDS system.