A thin layer of CdSe quantum dots (QDs) deposited on a conductive glass was used as a photo-anode in photo-electrochemical solar cells yielding a photovoltage of 675 mV and 2 mA cm-2 short circuit photocurrent. The response of the QD based photoactive electrodes correlated with the absorption spectra of the QDs which is easily tuned by size modification. Similar geometry based on acid treated CdS quantum dots, provided a photo-cathode with complementary photovoltage. A tandem photoelectrochemical cell consisting of the two photoactive electrodes exhibits Voc of 816 mV. The tandem configuration enables achieving the highest photovoltage reported to date for QD based photoelectrochemical cells. However, current matching still requires optimization of the spectral response and charge collection efficiencies of the two photoactive electrodes. The results demonstrate a new approach for the beneficial utilization of QDs in photo-electrochemical cells towards efficient, low cost photovoltaics.