The influence of the bubble diameter on the mass transfer coefficient and the reaction selectivity of a competitive-consecutive mechanism has been investigated by means of 3D Direct Numerical Simulations (DNS) of reactive mass transfer from deformable single bubbles. In order to assess the sensitivity of the transfer processes with respect to the chemical time scales, different chemical reaction intensities have been considered, covering the range from slow to fast reactions. Simulations have been performed using an Arbitrary Lagrangian Eulerian (ALE) Interface-Tracking algorithm, allowing for highly accurate hydrodynamic and reactive mass transfer results. With the concentration boundary layers being significantly thinner than the hydrodynamic boundary layer at the bubble interfaces, a two-mesh approach with a specialised refinement strategy has been adopted. In this approach, the reactive processes are numerically solved by using a second mesh with additional cell layers at the interface, alleviating the multiscale problem but keeping the computational costs reasonable. In the bubble diameter range investigated in this work, significant changes of the mass transfer coefficient and the reaction selectivity have been observed and discussed. Furthermore, the simulation results have been used to assess the validity of the film theory, also for technically relevant bubble sizes and reaction conditions. (C) 2017 Elsevier Ltd. All rights reserved.