Inert liquid tracer concentrations in a continuous mixer are analyzed by planar laser induced fluorescence (PLIF). Special attention is paid to the two separated entering feed streams containing the tracer solution or pure solvent. From the instantaneous tracer concentration fields, the method proposed allows one to easily calculate the instantaneous supersaturation fields, which would be obtained in the same mixing device with reagents instead of an inert tracer solution and pure solvent. A typical mixing situation in a stirred tank with separated feed streams is investigated. Maps of averaged supersaturation and averaged nucleation flux are yielded with high spatial resolution of a few tens of micrometers for each pixel. The method gives interesting indications about the ability of a given precipitator geometry to generate coarse or fine particles. However, it cannot be extended straightforward to partially premixed feed streams without the use of a mixing model. (C) 2003 Elsevier Science Ltd. All rights reserved.