Adaptable pervasive flows are dynamic workflows situated in the real world that modify their execution in order to adapt to changes in the execution environment. This requires on the one hand that a flow must be context-aware and on the other hand that it must be flexible enough to allow an easy and continuous adaptation. In this paper we propose a set of constructs and principles for embedding the adaptation logic within the specification of a flow. Moreover, we show how a standard language for web process modeling (BPEL) can be extended to support the proposed built-in adaptation constructs.