In this article we generalize the necessary conditions for a topological definition of the concept of molecular structure, showing that it may be independent of the particular criterion chosen, as long as it fulfills some general conditions and may give rise to the concept of a category of stable chemical structures. We proceed to introduce a local metric in the objects of this category by using the properties of metric space bundles, and this allows us to propose a general system of reaction mechanisms within this framework of the theory of categories. We conclude by proposing a single diagram that encompasses the whole theory of stable structures and reaction mechanisms. © 1992 John Wiley & Sons, Inc.