Product classification systems play a major role in 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 UN PSC 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 describing classification systems is missing so far. This paper discusses the design of classification systems and argues to develop standardized messages using XML chema for the transmission of classification systems.