The experimental study of gas dispersion in a vertical periodically, constricted, oscillatory meso-tube (OMT) is herein presented. Water was continuously pumped through the OMT in the laminar flow regime along with an oscillatory flow component superimposed into the net flow in a range of fluid oscillation frequency (f) and centre-to-peak amplitude (x(0)) of 0-20s(-1) and 0-3mm, respectively, in the presence of a very low superficial gas velocity (0.37 mm min(-1)). Bubble images were recorded with a CCD camera and analysed with Visilog((R)) software. A bimodal distribution of bubble size was in general observed but the bubble size was found strongly dependent on the oscillatory flow mixing conditions imposed into the fluid. A number fraction of micro-bubbles (with an equivalent diameter, D-eq, equal or bellow 0.2 mm) up to 60% was generated with increasing values of x0 (i.e. 3 mm) and values off in the range 10-15 s(-1). Furthermore, it is demonstrated that the Sauter mean diameter, D-32, and the specific interfacial area, a, can be fined tune by setting both f and x(0) in this studied range. The high number fraction of micro-bubbles was concluded to have a positive impact in enhancing the liquid-side mass transfer coefficient, k(L). Globally, the differences in bubbles sizes were found to play a marginal effect in the global enhancement of the k(L)a in the meso-tube in comparison with the intensive contact experimented by the bubbles rising in the oscillatory flow. The higher order of magnitude of the kL values found in this work (up to 0.0021 m s(-1)) is promising for running numerous industrial gas-liquid flows processes through smaller and better, while aeration of biotransformations can be run more efficiently, as supported by our recent proof-of-concept studies carried out in the platform. (c) 2007 Elsevier Ltd. All rights reserved.