Falling water films on completely wetted horizontal tubes of 16 mm diameter in a vertical row and the associated oxygen gas absorption were experimentally investigated in the Reynolds number range of 10 < Re < 150. It was found that the falling films form continuous sheets between the tubes when the tube spacing L-s is 2 mm, and that the film surfaces are smooth for Re < 30 whereas fine random waves appear on the surface for Re > 30. When L-s = 5 mm or larger, dripping from each tube occurs and causes the larger amplitude waves which rapidly spread on the film on the lower tube. The Sherwood number, Sh, greatly increases in L-s from 2 to 5 mm and then levels off at L-s = 10 mm or larger. When L-s = 2 mm, Sh is in proportion to Re-0.5 for Re < 30 and to Re-0.9 for Re > 30. The Sh varies as Re-0.86 for the whole Re range at L-s = 5 +/- 15 mm. Comparison of the present results with empirical equations for vertical tube absorber indicates that volume of horizontal tube absorber can be smaller than that of vertical tube absorber by a factor of 1/1.18, 1/2.2. (C) Elsevier Science Ltd. All rights reserved.