The compartmentalized anaerobic reactor (CAR) is a patent novel high-rate reactor and shows a great potential for its application. The hydraulic characteristics and their effects on the working performance of CAR were investigated. The flow pattern tended to plug flow at normal organic loading rate (OLR) and completely mixed flow at high OLRs. The relation of hydraulic dead space (HDS or V-h) with hydraulic loading rate (HLR or L)and biogas production rate (BPR or G) was V-h = 3.75L + 0.19G - 9.47. The hydraulic efficiency of CAR was good or near to good. Both HLR and BPR had significant effects on the hydraulic efficiency, but their effect became less at super-high OLR. They also had a slight influence on the effective volume ratios of CAR, but the influence of BPR almost disappeared at super-high OLR. The good working performance of CAR was ascribed to the improved reactor configuration. (C) 2012 Elsevier Ltd. All rights reserved.