Detailed, three-dimensional mass (heat) transfer distributions along four active walls of a square duct containing a sharp 180 degrees bend are presented. The duct simulates two passes of an infernal coolant channel in a gas turbine engine with inactive (insulated) ribs mounted on two opposite walls. Mass (heat) transfer measurements, taken using the naphthalene sublimation technique, are presented for Reynolds numbers between 5000 and 40000, for a rib-height-to-hydraulic-diameter ratio (e/D-h) of 0.1 and rib-pitch-to-rib-height ratios (P/e) of 10.5 and 21. Ribbed wall measurements show periodically developed mass transfer after three hydraulic diameters from the entrance, which agrees well with previous studies. Mass transfer distribution on ribbed walls has more span-wise uniformity than the smooth side-walls that experience high mass transfer rates close to the rib ends and near the corners downstream of each rib. The effect of the bend is clearly visible in the ribbed duct following the bend, The observed local Sherwood number distributions are explained on the basis of the secondary flow developed within the bend and possible separation from the inner wall after the bend. (C) 2000 Elsevier Science Ltd. All rights reserved.