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The provision of service differentiation is an important aspect that has to be considered for the definition of next-generation networks due to the high heterogeneity of the traffic that will dominate networks in the near future. This is particularly important in the context of optical burst switching, which is emerging as one of the strong candidate technologies for the next-generation optical Internet. Preemptive contention resolution schemes are very effective solutions for providing service differentiation in such networks; however, they cannot be applied together with the just-in-time signaling protocol because of the great loss in efficiency in terms of wavelength utilization and maximum achieved throughput that results when the number of preemptions becomes too large. This paper presents a preemption-based service differentiation solution that is suitable for the just-in-time optical burst switching paradigm because it keeps the preemption probability (i.e., the probability of observing a preemption when a contention occurs) low. Within the proposed technique, bursts are created at their ingress node and combined into chains, arranging them in order of decreasing priority. Then, a conventional preemption scheme is adopted at core nodes to handle contentions. An analytical study is presented and some traffic scenarios are analyzed by simulation to evaluate the performance of the proposed method.