Predictable performance is key for many WSN applications. Recent efforts use models of the environment, the employed hardware, and protocols to predict network performance. Towards this end, we present an intentionally simple model of ContikiMAC, Contiki’s default MAC layer, targeting worst-case bounds for packet delivery rate and latency. Our experiments reveal problems in the performance of ContikiMAC, which make the protocol perform much worse than predicted, and hence prohibit predictable performance with the current ContikiMAC implementation. We show that the reason for this performance degradation is that ContikiMAC loses phase-lock. To solve this problem, we add fine-grained timing information into the acknowledgment packets. We show that this mechanism solves these problems and enables predictable performance with ContikiMAC even under high external interference.