With the generalization of multi-core processors, dataflow programming is regaining a strong interest, especially in the context of compute intensive multimedia applications such as video decoding. How- ever, most studies focus on static approaches to the compilation and placement problems. We advocate for dynamic adaptation of dataflow applications. In this paper, we build the first step towards this goal, namely a monitoring mechanism for observing quality-of-service properties of programs at run- time. We propose a language extension for expressing simple QoS properties over dataflow programs together with a run-time mechanism for the observation of events meaningful to the QoS establishment. We show the limited impact of such mechanisms on the application overall performances.