Abstract Volumetric mass transfer coefficient has been estimated from the measurement of CO2 absorption in water for the slug flow in 2.2, 5 and 8 mm tubes. The results were compared with previous studies in the literature and analyzed with the penetration theory. The mass transfer coefficients in 5 and 8 mm tubes were substantially larger than predicted by a laminar film model. It was concluded that the waves at the gas–liquid interface significantly promoted the mass transfer in these tubes. A brief discussion is also given regarding application of the results as well as the effect of length scales on absorber performance.