The objective of this work is to study the flow behavior of crude oil in a battery of industrial crude oil/gas separators in oil industry. The battery is composed of three separators operating at different pressures. The residence time distribution (RTD) of the crude oil has been determined by an impulse injection of In-113 at the inlet of each separator and the concentration has been continuously recorded at the outlet. The real volume occupied by the crude oil has been determined by simple estimation of the first moments. The RTD of the crude oil has been simulated by a model composed of few mixing cells in series representing the effect of the deflector located at the entrance and a plug flow partly due to the high viscosity of the crude oil. The variation of the parameters of the model has been studied as a function of pressure conditions and they have been linked to the deposition of sediment. The tracer measurement in gas phase showed that this method can detect non-negligible malfunction but it is not sensitive enough to assume that the zero tolerance of oil concentration in gas phase requested by the exploitant is respected. (c) 2004 Elsevier B.V. All rights reserved.