Rotating biological contactors (RBCs) have been widely used for treatment of effluents containing soluble organic compounds. This study reports treatment of aqueous effluent containing diesel oil (0.6%) as a model non-aqueous phase liquid (NAPL) in a RBC. The NAPL serves as the sole substrate for the bacterial culture. Burkholderia cepacia that is inoculated along with a sessile algal culture for biofilm formation. After biofilm formation in batch mode, the 3-stage reactor was operated in a flow through mode. Data was collected over the unsteady state and also for steady state operation over hydraulic retention time (HRT) range 12-24 h and total petroleum hydrocarbon (TPH) loading range 23.9-47.8 g TPH/m(2) d. Greater than 99% removal could be achieved for TPH loading up to 31.8 g TPH/m(2) d. The models available for predicting removal efficiency of soluble substrate at various HRT values could not be applied for this system. While the assumption of uniform distribution in each stage of the RBC is valid for soluble substrate, it is invalid for NAPLs as illustrated by the unsteady state profile. The stage-wise effluent TPH profiles obtained during unsteady-state operation was indicative of a plug flow reactor with dispersion (PFDR) with sorption and biodegradation occurring simultaneously. The diesel oil in the aqueous phase sorbed on the algal-bacterial biomass and bulk of the aliphatic fraction was biodegraded. The aromatic fraction of diesel accumulated on the biofilm. This system is very complex and new models need to be formulated for understanding and elucidating treatment of NAPL containing aqueous effluents in RBCs. (C) 2012 Elsevier B.V. All rights reserved.