The formal step by step development of implementations from specifications is necessary to allow the incremental description of large software systems and hence split the software development process in manageable portions. Due to the complex notion of objects as units of structure and behavior, the refinement process has to be reconsidered in the object-oriented framework. Apart from refining structure the behavioral part gives rise to refine actions by transactions. Referring to information systems as application domain, concurrency control aspects come into play because of shared resources. We present an approach to}, longabstract={incorporate transactions into object-oriented specification and illustrate the main problems of synchronizing them on commonly used resources. The aim is that independent actions of different transactions, i.e., actions which are not accessing the same sources, may be arbitrarily interleaved. We envisage a denotational semantics based on event structures in which the sequential composition of transactions can be appropriately liberalized and outline the ideas by giving some examples.