Fiber suspensions of different types of wood pulp were studied by using the pulsed gradient spin-echo NMR technique. Water in pores inside the fibers can be discriminated from bulk water by using this method, since the diffusion of fiber water is restricted when the diffusion time is sufficiently long. The amount of fiber water in a suspension is linearly correlated to the fiber concentration of the suspension. From this linear correlation, the average pore volume of the pulp fibers studied was rigorously estimated to be 1.6 +/- 0.3 cm(3)/g of cellulose fibers, which independently confirmed the results from solute exclusion studies. Furthermore, the self-diffusion coefficients of bulk water and fiber water were also determined. The self-diffusion coefficient of bulk water shows a concentration dependence due to the obstruction and hydration effects, but the self-diffusion coefficient of fiber water is constant in the concentration range studied.