Traditionally the characterisation of mixing in a system is achieved by injecting a tracer solution and monitoring the changes of tracer concentration with time using conductivity probes. In this paper we demonstrate that a non-intrusive laser-induced fluorescence (LIF) technique can be used to quantify the behaviour of viscous fluids in a model oscillatory baffled column (OBC). The OBC is an alternative-mixing device in which longitudinal periodic fluid oscillations are transposed onto a column with evenly spaced 'doughnut' shaped baffles. This combination of baffles and oscillation generates an enhanced yet uniform mixing system. In LIF, a dye fluoresces when it is induced by a laser. The intensity of the light emitted can be converted into the concentration of the dye. The timed profile of average concentration (within a baffled cell) leads to the determination of the mixing time and axial dispersion coefficient. This paper attempts to show that LIF is a powerful tool in studying mixing. (C) 2003 Society of Chemical Industry.