A PLIF technique is described which enables the measurement of mixing performance in a 0.06 m square section vessel as the vessel is filled using a vertical dip pipe. The liquids used were Newtonian aqueous solutions of glycerol (mu = 3-12 mPa s); the values of Reynolds number of the entry jet ranged from 343 to 3,735. The vessel was initially charged with a small volume of fluorescent tracer, and a vertical plane passing through the vertical axis of the vessel was illuminated using a laser sheet. The fluorescence emitted by the dye was detected using a CCD camera and a calibration routine was developed to relate the measured fluorescence to the local dye concentration. The technique is shown to be capable of determining the mixing mechanism via visual inspection whilst log-variance (quantitative) analysis was employed to determine the degree of mixedness as a function of the Reynolds number and energy input per unit mass.