Direct numerical simulation results of mass transfer in dense bubble swarms using a Front Tracking (FT) model will be presented, where the effect of the gas hold-up has been investigated. The FT method is particularly suited for bubble swarm simulations, since bubbles do not coalesce artificially, but traditional FT techniques often suffer from artificial volume loss of the bubbles. For this reason, a specialized remeshing technique is presented to counteract any occurring volume defects, while keeping all physical undulations on the bubble surfaces unharmed. For the simulation of gas-to-liquid mass transfer, a species transport equation (convection diffusion -reaction) was coupled to the FT hydrodynamics solver, which was solved on a superimposed refined mesh for higher accuracy. The velocity components have been interpolated to the refined grid using a higher-order solenoidal method. Enforcement of the Dirichlet condition for the concentration at the gas-liquid interface is achieved with an immersed boundary method, enabling the description of gas to liquid mass transfer. Careful validation of the newly implemented model shows satisfactory results. The liquid side mass transfer coefficient in dense bubble swarms, with gas fractions between 4% and 40%, has been investigated using the new model. The simulations have been performed in a 3D domain with periodic boundaries, mimicking an infinite swarm of bubbles. The results indicate that the liquid-side mass transfer coefficient rises only slightly with increasing gas fraction. (C) 2016 Elsevier Ltd. All rights reserved.