In a previous paper it was shown that FAD in aqueous solution decomposed into riboflavin-4', 5'-cyclic phosphate and adenosine-5'-phosphate when it was kept for a long time. It was found that the decomposition of FAD was greatly accelerated by the presence of trace amounts of heavy metal ions which were brought into the solution from the glass container. The FAD was more stable in the polyethylene container than in the glass. The decomposition of FAD in water was inhibited by adding a small amount of ethylendiaminetetraacetate.