Classical approaches to quality-of-service (QoS) provisioning in IP networks are difficult to apply in all-optical networks. This is mainly because there is no optical counterpart to the store-and-forward model that mandates the use of buffers for queuing packets during contention for bandwidth in electronic packet switches. Since plain IP assumes a best effort service model, there is a need to devise mechanisms for QoS provisioning in IP over wavelength-division- multiplexing, or IP-over-WDM, networks. In this paper we propose a new scheme, called Preemptive Prioritized Just Enough Time (PPJET), for QoS provisioning in buffer-less optical burst switching (OBS) networks. PP-JET provides strict priority for high priority traffic by dropping reservations belonging to lower priority traffic using a new channel scheduling algorithm called Preemptive Latest Available Unused Channel with Void Filling (PLAUC-VF). Furthermore, we study the performance of PPJET through simulation experiments, and we show that PPJET outperforms Prioritized Just Enough Time (PJET) in terms of dropping probability and end-to-end delay.