While BPEL language has emerged to allow the specification of Web Service compositions from a functional point of view, it is still left to the architects to find proper means to handle the Quality of Service (QoS) concerns of their compositions. Typically, they use ad-hoc technical solutions, at the message level, that significantly reduce flexibility and require costly developments. In this paper, we propose a policy-based language aiming to provide expressivity for QoS behavioural logic specification in Web Service orchestrations, as well as a non-intrusive platform in charge of its execution both at pre-deployment time and at runtime.