Entrance effects on the Sherwood number are investigated in a narrow rectangular channel, both with ribbed walls and filled with different mesh-type spacers, and for Reynolds numbers, based on the channel height, varying between 5 and 500. Mass transfer coefficients were measured by the limiting current technique, using the ferri-ferro cyanide redox couple with 0.5 M potassium carbonate in a test cell with a narrow rectangular channel (15 mm x 2 mm x 170 mm) and 8 consecutive segmented nickel electrodes (11.2 mm long and 10 mm wide) fitted in the bottom wall. Five mesh-type spacers made of two layers of parallel filaments (1 mm diameter) and having different flow attack angles were tested. The ribbed walls tested had transverse square ribs (1 mm x 1 mm) uniformly spaced, with inter-ribs distances of 3.8, 7.6 and 11.35 mm. Average Sherwood number for each electrode was measured for both cases of the upstream electrodes being active (under limiting current conditions) or inactive (without current). Results show that for stable laminar flow regime strong entrance effects can prevail over the entire channel whenever transverse or oblique filaments are not in contact with the electrodes. Diamond-shape spacers presented no mass-transfer entrance effects even at a lowest Reynolds number of 5. In the transitional flow regime, i.e., at Re above the critical value, the Sherwood number decreases sharply in the first periodic channel segments with posterior stabilization for all the spacer configurations tested. This work gives guidelines for using periodic boundary conditions in the CFD simulation of flow and mass-transfer inside spacer-filled narrow rectangular channels with relevance for membrane modules design. (C) 2012 Elsevier Ltd. All rights reserved.