Experiments are performed to determine the response of the pressure drop and local heat transfer on the shell side of shelf-and-tube heat exchangers to a change in the leakage between baffles and shell in the fully developed regime. The local mass and heal transfer coefficients are visualized and determined by means of mass transfer measurements. The shell-side flow distribution was calculated from the measurements of the pressure distribution in the baffle compartment. This leakage between baffles and shell can greatly reduce the pressure drop and the per-compartment average heat transfer coefficient. (C) 1997 Elsevier Science Ltd.