The stability issues of CdSe quantum dot-sensitized solar cells have been investigated in this work. Stability is one of the most relevant issues for the real-world applications of quantum dot-sensitized solar cells. We find that CdSe quantum dots deposited in air are prone to oxidation, and when the solar cells are left in air for several months, CdSe quantum dots decompose partially to form Se particles and nanoribbons. The surface oxidation and the decomposition of CdSe quantum dots worsen all the parameters of the solar cells. Electrochemical impedance characterization provides unambiguous evidence for the evolution of the cells in the ageing process, which accounts for the observed performance worsening phenomena. Preparation under an inert atmosphere suppresses to some extent the degradation of the cells. These findings have significant implications for designing viable, high-performance quantum dot-sensitized solar cells.