Some of the important characteristics of services environments are being dynamic, distributed, loosely coupled and open. These characteristics result in the existence of different levels of functional and non-functional properties of the services operating in such environments. Consequently, it creates challenges for interacting service consumers that require to only deal with services that are trustworthy. In service compositions, the component services may be mandatory or optional and vary in their contribution to the trustworthiness of the composite service. Composition techniques must be able to select trustworthy components and to dynamically adapt to subsequent changes in the services and the environment. The availability of multiple services providing the same or similar functionality but with different trustworthiness levels helps composite service providers to establish and maintain trustworthy compositions. The paper describes an approach to the aggregation of trustworthiness properties of composite services that are assembled from more fine-grained component services. The approach is developed for composite services based on BPMN business processes but can be extended to other languages.