Moisture has a profound effect on the physical properties of paper and thus determines the performance characteristics of paper products including the so-called mechanosorptive effect where the creep behavior of paper is accentuated by cyclic humidity variations. Moisture transport in a paper sheet exposed to ramp changes in the external humidity has been studied. Our transport model considers a paper sheet as a composite medium of fibers and void spaces with moisture transported by diffusion through each one. Diffusion in the fibers is approximated by a lumped linear relaxation process. The important dimensionless parameters consist of the Blot, fiber, and void diffusion numbers. A comparison of the model predictions with experimental transient moisture uptake of a bleached kraft paper board sample showed that a single value of the fiber relaxation parameter, k(i), was sufficient to give good estimates of transient moisture sorption data.