A model of simultaneous heat and mass transfer through a porous material is presented to explain transient moisture sorption by paper sheets from humid air. There are three primary resistances to moisture transport: (1) diffusion through an external boundary layer; (2) diffusion through the pore system and, (3) diffusion from the pore system into the fibers. It is found that diffusion through the fiber phase perpendicular to the plane of the sheet is not significant for the moisture content range considered here. The mass transport model is able to predict the results of transverse moisture gradient experiments which show that moisture content gradients in paper are not as large as previously thought during transient periods. The model shows that the sigmoidal temperature response of paper to a linear change of relative humidity is due to non-linearities of the moisture content isotherm and heat of sorption. (C) 2003 Elsevier Science Ltd. All rights reserved.