Minium (Pb₃O₄) and cinnabar (HgS) were commonly used as red pigments on paintings in ancient times. The discoloration of these red pigments has been a long-term concern for conservators, curators and art historians. However, it is difficult to study the degradation mechanism of these historical materials because of their long time exposure to the environment and complex decaying factors. To investigate and monitor the process of photodegradation of minium, cinnabar and their mixtures, an electrochemical setup with a pigment-modified graphite electrode was used. It was confirmed that light irradiation of minium induced a reductive photocurrent, while an oxidative photocurrent was detected for cinnabar. In this work, the photochemical reaction of the mixed pigment of minium and cinnabar was determined to be a reduction process. Both pure and mixed pigments having experienced ultraviolet aging were analyzed by Raman and X-ray photoelectron spectroscopy (XPS). Minium was partly converted to cerussite (white lead) and hydrocerussite after UV light aging. The reaction kinetics results show that the addition of cinnabar into minium might accelerate its photodegradation reaction.