Sodium hyaluonate (NaHA) and Methylene Blue (MB) formed a complex through charge transfer (CT) interaction or ion association in aqueous solution. Irradiation of the complex using a 313-nm light under argon caused degradation of NaHA. The mechanism proposed on the basis of the effect of excitation wavelength and oxygen, and by comparison with the degradation of NaHA using Rose Bengal (RB) and TiO2, was that photoinduced electron transfer from NaHA to upper excited state MB initiated the degradation.