The zero length column (ZLC) technique has been successfully used to measure diffusivities in zeolite crystals. However, in industrial applications pellets with bidisperse structure, containing macropores and micropores (crystals), are commonly used as zeolites. In this paper, a model of ZLC desorption curves for bidisperse porous materials is developed. Model equations are analytically solved for linear systems. A numerical solution of model equations using orthogonal collocation is also used. The various regions of control (macropore diffusion, micropore diffusion) are identified. Simulations allow the choice of operating conditions for the ZLC technique. Procedures for the analysis of ZLC experiments are reported.