The efficiency of biological wastewater treatment systems is linked fundamentally to the hydraulic performance of each treatment unit. These units should guarantee an adequate contact between the microorganisms and the residual water, and the compliance with the hydraulic retention time established, in order to decrease the number of dead zones or short circuits that may exist inside the reactors. In this work, hydraulic performance was evaluated in seven Expanded Granular Sludge Bed (EGSB) reactors with a useful volume of 3,4 L and constructed in acrylic. The analysis was carried out through the stimulus-response test, using bromide as tracer. Two hydraulic retention times (8 and 24 h) and the effect of the presence of granular biomass were considered. Results were analyzed qualitatively through the construction of curves C, E and F, and quantitatively through the construction of a mathematical model of axial dispersion. The results of the hydraulic performance of the reactors revealed a marked tendency to a complete mix flow pattern, with a low effect of HRT or the presence of granular biomass on their operation.