Wave evolution in thin-film flows is highly relevant for heat and mass transfer applications, such as CO 2 capture in falling film absorbers. To develop a detailed understanding of potential enhancement mechanisms associated with the evolution of three-dimensional (3D) waveforms, we perform 3D direct numerical simulations of passive scalar transport in laminar-wavy film flows, using a hybrid front-tracking/level-set method to accurately resolve interfacial features. CO 2 absorption is greatly enhanced in the presence of interfacial waves with the liquid-side mass transfer coefficient increasing tenfold relative to that of a flat film for the highest film Reynolds numbers ( R e ) studied. This is primarily due to changes in interfacial and internal flow dynamics rather than an increase in the gas-liquid interfacial area. The recirculation region present in the leading and trailing fronts of the 3D waves intensifies mass transfer, and their effectiveness increases with R e . At low R e , there is a film region beneath the wavy interface, which remains relatively undisturbed where mass transfer is dominated by diffusion. The introduction of structured substrates to promote mass transfer under these conditions is recommended. The visco-capillary ripple region, which precedes the leading and trailing fronts for sufficiently high R e , provides a relatively high degree of spanwise advection, with the mean spanwise velocity magnitude reaching around one-quarter that in the streamwise direction. This underscores the importance of solving the fully-3D problem as these effects do not have a two-dimensional analogue. • Direct numerical simulations of 3D falling films with CO 2 mass transfer. • 3D interfacial waves enhance CO 2 absorption. • Recirculation zones in wave fronts intensify mixing at high Re. • Flat film region limits CO 2 absorption at low Re. • Spanwise advection in ripple region is significant at high Re.