With orchestrations, one service may be realized through the cooperation of several services. This cooperation has to be formally described. In this paper, we propose to describe service orchestrations according to UML2 meta-model through three UML2 diagrams. Component diagrams describe each service external interfaces. Collaboration diagrams describe the structural composition of services. And activity diagrams describe the orchestration of services. The main contribution of this article is to mix the orchestration and composition meta-model with a context meta- model. Thus, we propose to include the descriptions of context awareness into the orchestration and composition meta-model. This approach allows application designers to describe flexible orchestration of services. Furthermore, describing compositions and their context-awareness with a model (conform to a meta-model) allows middleware with model-transformation capabilities to produce ad-hoc compositions in term of adaptation to current context execution and in term of target execution platforms. We present in this article two kinds of adaptation of context aware orchestrations: deployment time and run time adaptations.