Riboflavin (RF), photosensitizer, was used in combination with irradiation with light of wavelengths in the range of 310-800 nm to induce degradation of the biopolymer alginate. The effects of molecular weight and concentration were studied. The viscosity increases as the concentration is raised both for the alginate and alginate-RF solutions but is lower for the solutions irradiated in the presence of RF. Similar to many entangled and/or associated polymer networks this system shows strong shear-thinning effect at higher concentrations. The viscosity change induced by illumination in the presence of RF seems to be most extensive for the solution with the highest molecular weight. This was emphasized by oscillatory shear measurements, which indicate that RF in combination with irradiation has most effect on the theological parameters of alginate of the highest molecular weight.