An anaerobic expanded granular sludge bed (EGSB) reactor is considered to be an improvement over upflow anaerobic sludge blanket reactors owing to the former's ability to recycle the effluent and its modified reactor geometry. However, the mixing pattern in EGSB reactors, which greatly influences the design and the performance of this reactor, has not yet been studied in detail. In this research, the mixing pattern in a lab-scale EGSB reactor treating a synthetic dye wastewater was studied using lithium chloride as a tracer. The tracer exit curve indicated a complete-mix behavior. A simulation study was conducted on identical reactors using conductivity probes, inserted through the sample ports along the height of the reactors and connected to a data acquisition system. The reactors were operated at three different hydraulic retention times (3.3, 5.5, and 9 h) and at four different upflow liquid velocities (1.10, 2.66, 5.33, and 8.68 m/h). The data showed the existence of a plug-flow regime in the basin at lower upflow liquid velocities although the tracer response curves resemble complete-mix behavior. With increasing upflow liquid velocity the flow pattern in the basin deviates from a plug-flow pattern and approaches a complete-mix condition. The EGSB reactor can be modeled as a plug-flow reactor with recycle and dead space, and with a large vessel dispersion number (D/uL>0.01). © 2010 ASCE.