An on-line in-situ technique has been developed for assessing mixing quality in pulp fibre suspensions in laboratory equipment. A fibre optic probe measures the concentration of an inert ultraviolet tracer added to the suspension at the ppm level. The method is able to resolve mixing quality at the centimetre scale (10 mm) and gives mixing indices that agree with those obtained using conventional techniques. The method allows assessment of mixing dynamics as demonstrated for laboratory batch mixing. As the fluorescent tracers evaluated react with typical bleaching chemicals it would be difficult to apply the technique in the bleach plant. However it could be used as a diagnostic tool in other non reacting mill environment.