A mechanistic kinetic model has been developed for the hydrocracking of complex feedstocks such as vacuum gas oil (VGO), based on an exhaustive computer-generated reaction network of elementary steps. The model utilizes a detailed composition of VGO, characterized by 16 different molecular classes up to C-40. These classes are divided into 45 subclasses by distinguishing the isomers of a class according to the number of methyl branches, leading to 1266 groups of isomers/pure components. The frequency factors of the rate coefficients for the acid site steps are modeled using the single-event concept, and the activation energies are modeled based on the nature of the reactant and product carbenium ions. The saturation of polyaromatics is modeled according to the sequential hydrogenation of aromatic rings on the metal sites. The competitive chemisorption of aromatics and naphthenes is taken into account, based on the number of aromatic/naphthenic rings on the metal sites, and based on the corresponding gas-phase proton affinities on the acid sites. The model contains 33 feedstock and temperature-independent parameters that have been estimated from experimental data on VGO hydrocracking. The kinetic model is inserted into an adiabatic multibed trickle-flow reactor model. The vapor-liquid equilibrium coefficients in this model are calculated using the Peng-Robinson equation of state. The model has been used to study the effect of operating conditions on the product profiles along the reactor. An analysis of the distribution of isomers of a class among its different subclasses shows the increase in VGO conversion when the content of isomers with a higher degree of branching is increased in the feedstock. This indicates the necessity of dividing a class into four subclasses.