Business processes can use Internet of Things (IoT) information to monitor context data in real-time and to respond to changes in their values in a timely fashion. For this matter, business process definition and execution languages should foresee an easy way for process modelers to define which values to monitor, and which automatic behaviors to adopt when these values change. In this paper, we propose the use of context variables to monitor sensor values, as well as a when-then language construct to detect and handle changes in these values within business processes. We define a Web Services Business Process Execution Language (WS-BPEL) extension to convey these constructs, and implement then using a "BPEL language transformation" approach. With these contributions, process modelers can define IoT-aware business processes avoiding the increase of process complexity and keeping their focus on modeling the processes' main logic. In addition, the language transformation approach assures the portability of processes using our constructs amongst WS-BPEL execution engines.