The evolution of the concentration profiles of an adsorbing solute diffusing in a stagnant bed of cellulosic fibers is measured using magnetic resonance imaging. Effective diffusivities are calculated by matching concentration profiles with a numerical model of one-dimensional Fickian diffusion. The measured values of the effective diffusion coefficients are interpreted using a model that accounts for the porosity, tortuosity, and adsorption equilibrium constant. The effective diffusivities in fiber beds are significantly lower than the bulk diffusivity of the solute and highly dependent on fiber type. The diffusivity is lower for the fiber type that exhibited stronger adsorption properties. The influence of concentration, adsorption, and other fiber characteristics on diffusion rates and rates of reaction is discussed. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 59-68, 2012