Volatile organic compounds (VOC) are major indoor air pollutants. Physical models that have been developed to predict VOC source (emission) and sink behavior (sorption) of building materials, commonly adopt the conventional convection approach using third-kind boundary condition assuming quasi-steady convective mass transfer in fluid (air). In this study, conjugate mass transfer models were developed to investigate the validity and application limit of the assumption. The conjugate mass transfer models consider either quasi-steady or unsteady two- dimensional laminar forced convection over a flat plate coupled with unsteady one-dimensional diffusion and sorption within the porous solid through the concentration and the flux continuities at the solid-air interface. The simulation results indicated that the quasi-steady assumption in convection mass transfer may result in significant overestimation of wall concentration or underestimation of Sherwood number in the early transfer phase; however, the errors caused by the quasi-steady assumption become less than 5 % after about 2.5 times of convection characteristic time, i.e., the time taken by a particle moving with a constant velocity to travel along the plate.