A thin layer of chlorophyll a (around 2000A). a p-type organic semiconductor. was sandwiched between two different metals. aluminum and silver. We used the photovoltaic effect in order to study the efficiency of light conversion by the crystalline and anhydrous forms of Chla. When the photovoltaic cell is illuminated through thc semi-transparent aluminum electrode. an action spectrum similar to the visible absorption spectrum of Chla is obtained. The anhydrous form. always shows a maximum, in the red, at 672 nm and the crystalline one at 746 nm to 738 nm depending on the amount of water vapor present in the measuring area. The light conversion efficiency has been measured at the maximum absorption in the rcd for both forms of Chlu. For the anhydrous form, we found values as high as 0.036"<, which is very significant even compared to 0.21'),,. the highest value found for the crystalline form. In both cases. the incident light power was approximately IOpW on the cell. The light conversion efficiency of both forms of Chla cells was found to depend on a combine effect of the nature of the gas (O1. N2. air. Ar. N,O. SF,. HZ) and the amount of water vapor present in the measuring area. The best conversion efficiency was obtained with 0, or air saturated with water. This combined effect was very large for the crystalline form whereas the anhydrous form was only slightly affected. INTRODUCITON