The capacitance measurements of Al/Chlorophyll a/Ag and Al/Chlorophyll a-Sulfoquinovosyldiacylglycerol/Ag sandwich cells have been carried out at different frequencies in dark and under illumination. The results show that while a voltage-dependent capacitance and a linear 1/C-2 versus V-a plot is obtained for chlorophyll a (Chi a) at low frequencies, the addition of sulfoquinovosyldiacylglycerol (SQDG) in proportions similar to 0.25 results in totally voltage-invariant capacitance. The capacitance characteristic of Chi a-SQDG (0.25) cells resemble more those for an insulator than those for a Schottky barrier. These results are explained in terms of negatively charged SQDG playing a role of traps which immobilize the holes, Alternatively, the interaction of negative polar head group of SQDG with Mg2+ of Chi a can also be invoked as a possible reason for the observed results, The: bound Chi a- SQDG species possibly does not form a Schottky barrier or forms a weak barrier with either Al or Ag electrodes, resulting in insulator-like capacitance characteristics of Al/Chl a-SQDG(0.25)/Ag cells.