The hydraulic characteristics of an anoxic rotating biological contactor were studied under different flow rates. The experiments were carried out with the reactor clean (without biomass) and containing denitrifying biofilm (Alcaligenes denitrificans) covering the disks. Residence Time Distribution (RTD) experiments were performed by the stimulus-response technique using lithium chloride as tracer. Experiments without biomass revealed the existence of hydraulic dead volumes (around 40% for hydraulic residence time of 0.94 and 2 hours) that occur in corners, where stagnant eddies form. When in normal operation, with the disks covered by biofilm and with biogas production, these values decreased significantly. For hydraulic residence time (HRT) of 2 hours a minimum dead volume was observed, being appropriate to run the reactor under this condition, from the hydraulic viewpoint. The Dispersion number decreased with increasing HRT from 0.94 h on, for both types of experiment, without, and in the presence of biomass. For this HRT the dispersion number was maximal in both situations. A considerable diffusion of tracer into the biofilm was detected, being faster in the more hydrated biofilm, and Justifying the long tails observed in the RTD experimental curves.