In the context of information system engineering, we propose a four-layer metamodeling architecture with a comprehensive set of operations on metamodels. Our architecture enables modelers to use a three-step modeling process: first, giving an informal description of the universe of the discourse (in terms of modeling paradigms); then, defining a corresponding UML dialect (in terms of metamodels); and finally "using the chosen dialect" describing a model of an information system. By using speci.c properties of our metamodeling architecture, we define formal and semantical operations on metamodels, e.g., integration of metamodels. In this paper we focus on a measure of a semantical distance between metamodels.