The advent of Service Oriented Architectures tends to promote a new kind of software architecture where services, exposing features accessible through highly standardized protocols, are composed in a loose coupling way. In such a context, where services are likely to be replaced or used by a large number of clients, the notion of Quality of Service (QoS), which focuses on the quality of the relationship between a service and its customers, becomes a key challenge. This paper aims to ease QoS management in service compositions through a better separation of concerns. For this purpose, we designed QoSL4BP, a domain-specific language which allows QoS policies specification for business processes. More specifically, the QoSL4BP language is designed to allow an architect to specify QoS constraints and mechanisms over parts of BPEL compositions. This language is executed by our ORQOS platform which cooperates in a non-intrusive way with orchestration engines. At pre-deployment time, ORQOS platform performs service planning depending on services QoS offers and on the QoS requirements in QoSL4BP policies. At runtime, QoSL4BP policies allow to react to QoS variations and to enact QoS management related mechanisms.