The objective of this research is to compare empirical (pseudo-first and pseudo-second order models) and diffusional models (film diffusion, film-pore-volume diffusion, and film-surface diffusion models) in predicting the adsorption kinetics of chromium(III) on water-washed agro-waste materials (sorghum straw, oats straw, and agave bagasse). Concentration decay curves can be predicted by using either empirical or diffusional kinetic models. However, the film diffusion model seems to be the most appropriated one based on the low reported deviation (0.45-4.09%), and the physical properties (low porosity 0.004-0.007, low surface area 0.6-1.2 m(2) g(-1) and low pore volume 0.003-0.004 cm(3) g(-1)) of the studied agro-waste materials, which support the idea that intraparticle diffusion may be neglected. Furthermore, the external mass transfer coefficient estimated with the film diffusion model has a physical meaning that helps to explain the diffusion of solutes across the film resistance in agro-waste biosorbents. (c) 2010 Elsevier Ltd. All rights reserved.