A particle image velocimetry (PIV) autocorrelation technique was applied to produce whole field vector maps of flow beneath and to the side of a 17 row x 17 column kettle reboiler thin slice rig, boiling pentane at atmospheric pressure. The flow proved to be time dependent. The average values of the mass flowrates of the recirculating liquid were evaluated at various positions and compared to predictions of models of the flow which reproduced the measured pressure drops [B.M. Burnside, K.M. Miller, D.A. McNeil T. Bruce, Heat transfer coefficient distributions in an experimental kettle reboiler thin slice, Trans. JChemE 79A (2001) 445-452]. The use of the results as a platform for 2D numerical modelling of the flow is emphasised. (C) 2004 Elsevier Ltd. All rights reserved.