Man has had an intense interest in the role of enzymes in biological reactions for many years. The basic question--how enzymes are able to function so efficiently--remains as yet largely unanswered. Only in recent years has it been discovered that molybdenum is essential for certain biological processes. For example, traces of molybdenum added to certain soils have resulted in spectacular plant growth (1). Four enzymes are known to contain molybdenum: nitrate reductase (2), hydrogenase (3), xanthine oxidase (4), and aldehyde oxidase (5). In each of these enzymes riboflavin is also present as the coenzyme FAD (flavin adeninedinucleotide). Tocatlian, in 1960, showed that a strong complex containing two molybdenum atoms per riboflavin molecule is formed in solution. However, he was unable to determine the formation constants (6).