Flow and heat/mass transfer in a plate fin and two-tube assembly is examined using the naphthalene sublimation technique. The examined parameter is the offset of two-tube centers to tube diameter ratio s/D for two Reynolds numbers Re-D: 1770 and 2660. The local heat/mass transfer coefficient is always large at the leading edge of the plate and also in front of the tube. The horseshoe vortex formed in front of the first tube increases the local heat/mass transfer rate not only around the first tube itself but also around the second tube. The total heat/mass transfer rate from the plate increases with s/D and it reaches a saturation beyond s/D = 0.5. Similar behavior is observed for the pressure drop, showing that s/D congruent to 0.5 is the optimal offset for the tested range. When the Reynolds number is as high as 2660, the effect of tube offset is significant, so that the total heat/mass transfer rate from the plate for the two-row tube case is larger than that for a single tube case. (C) 2003 Elsevier Science Ltd. All rights reserved.