Standardized product classification systems play a major role for searching and comparing offered products on electronic markets. Especially in case of large multi-vendor product catalogs classified data becomes an important asset and success factor. The most known systems are UNSPSC and eCl@ss, however they are still developing, and new systems are emerging as well. Classification systems differ not only in content but also in structure from each other. The management and exchange of the systems between market partners must be able to get along with these differences. A common structure model, which can be used to specify XML business documents, is missing so far. This paper discusses the design of classification systems and develops a data model using XML Schema. The model can be used for the transmission of classification systems, thus it is an innovative extension of existing product catalog standards.