The bonding of hydrogen in the Al13H aggregate is analyzed in the framework of density functional theory using the local density approximation. The interaction between the H-1s orbital and only certain molecular orbitals of Al-13 is responsible for the binding. Different measures of the charge transfer give consistent results and predict the stabilization of a sizable amount of electronic charge, about two electrons, around the proton site. The state of the H atom can be described as a negatively charged impurity screened by the surrounding electron gas, similarly to a H impurity embedded in a vacancy in metallic aluminum. Friedel-type oscillations can be appreciated in the screening charge. Local Fukui functions and condensed Fukui indexes associated to the ground state of the cluster Al-13 are used as indicators of molecular reactivity. Those indices allow to predict and understand the equilibrium location of H found in the total energy calculations for Al13H. (C) 2003 American Institute of Physics.