The concept of web services provides some kind of middleware for the implementation of distributed applications, independent from platforms and programming languages. When developing new software systems, it could be of use in terms of time and costs to re-use the functionality of existing services. This process of re-use is called web service composition, but there are still some open topics currently under research. Beneath the matching between semantic description of offered and requested services, also the capabilities of the composed service are of importance. The current web service standards are not equipped to consider non-functional requirements, i.e. QoS aspects, of a user to a composed service. This topic is treated in this paper. An approach is presented how to enhance the web services architecture to consider QoS aspects in the composition of a complex service. Basing on aggregation of QoS descriptions assigned with simple services due to a workflow pattern of a composed service, a QoS broker is implemented which completes the directory service UDDI by mediating composed services in a QoS-based way.