A mathematical simulation carried out to study the physical mechanisms of moisture diffusion into hygroscopic fabrics during humidity transients is reported. On the basis of a mathematical model developed to describe the coupled heat and moisture transfer in wool fabric, the moisture-sorption mechanisms are investigated for fabrics made from fibers with different degrees of hygroscopicity. Theoretical predictions on the moisture uptake and temperature changes under humidity transients are compared with those measured previously in a sorption-cell experiment for fabrics made from wool, cotton, acrylic fiber, and polypropylene fiber. It is concluded that the physical mechanism of moisture diffusion into highly hygroscopic fibers such as wool and cotton can be described by a two-stage moisture-diffusion process: a fast Fickian diffusion with a concentration-dependent diffusion coefficient and a slow diffusion with a time-dependent diffusion coefficient. For weakly hygroscopic fibers such as polypropylene fiber, the moisture-sorption process can be described by a single Fickian diffusion with a constant diffusion coefficient. Through theoretical calculations of the distributions of moisture concentration in the air of fabric void space, fiber moisture content, and temperature through fabric thickness, we show that moisture diffusion into a fabric through air is a fast process for all the fabrics studied. Meanwhile, the moisture diffusion into fibers is coupled with the heat-transfer process, which is much slower and is dependent on the ability of fibers to absorb moisture. The strength of the coupling effect is a function of a number of fiber properties, such as the moisture-sorption isotherms, water-diffusion coefficient, fiber diameter, and heat of sorption.