Flavonoids (derivatives of 2-phenylo-y-benzopyrone) are a large group of biologically active plant products. As a common component of daily nutrition, flavonoids can modulate many biological processes. Therapeutic properties of flavonoids have been well known for many years. Originally, the object of interest was medicinal plants. The papers by Szent-Gyorgy in the thirties of the 20-th century aroused interest in flavonoids. Szent-Gyorgy revealed that a lemon compound belonging to flavonoids - hesperidin has a synergic effect with Vitamin C and prevents scurvy. Quercetin (3,5,7,3',4'-pentahydroxyflavone) and morin (3,5,7,2',4'-pentahydroxyflavone), Fig. 1, are the most widespread flavonoids. Due to their properties these compounds find application in therapy as viral antigens, bactericide and antitumor agents. Owing to a convenient position of oxygen in the 5C-OH and 4C=O as well as 3C-OH and 4C=O groups, morin and quercetin form chelate complexes with ions of p-, d- and f-electron metals. Some researchers think, that biological activity of flavonoids is connected with their complexation capacity. Therapeutic action can show both the molecule and its metabolite. The application of quercetin and morin is restricted due to their insolubility in water. Their sulfonic derivatives are well soluble in water and show biological activity as their mother compounds. Therefore, the latter can be more useful for therapy purposes.