Abstract. We investigate the behaviour of an asynchronous single-wavelength fiber delay line (FDL) buffer by means of a continuous-time queueing model. Through a limit procedure, previously obtained results for a discrete-time queueing model are translated to a continuous-time setting. The two important cases of exponentially distributed burst sizes and deterministic burst sizes are discussed in detail. The performance of asynchronous optical buffers proofs to be similar to that of synchronous optical buffers: a reduction in throughput due to the creation of voids on the outgoing channel and a burst loss probability that is strongly influenced by the choice of FDL granularity. The optimal value of the latter depends on the burst size distribution and the offered load. Synchronisation is found to be advantageous, yielding a lower loss than the asynchronous case for the same offered load.