Abstract In this paper a kinetic model for the residue hydrotreating is presented. The model considers kinetics, adsorption and diffusion inside the hydrodemetallization catalyst pellets. The diffusion is described by the Stefan–Maxwell equations extended for starkly different sized molecules, due to molecular size of residue molecules large distribution. Effective diffusion coefficients vary with porosity and tortuosity evolution due to volume constraints in the catalyst pellets. The model's kinetic and thermodynamic parameters were estimated from a set of experiments carried out in a batch reactor with a Middle East vacuum residue. A comparison between experimental and simulated metal profiles inside the catalyst exhibit the slow diffusion, validating the importance of adding mass transfer phenomena and considering volume constraints even in a macroporous demetallization catalyst. The model was validated by other tests performed in the same reactor using the same vacuum residue, but with different catalysts. The differences in the catalyst pore size allowed to predict all the hydrotreatment yields, showing diffusion is a crucial factor.