The influence of steric interactions in the initial stages of aggregation kinetics in colloidal systems has been studied by simulation and experiments. A simulation model has been proposed to study the initial stages of aggregation under short range repulsive potentials. A polystyrene latex was used as model colloid and the steric interaction was provided by adsorption of a nonionic surfactant. Depending on the range or strength of the interactions, sensitive or insensitive systems to electrolyte concentration can be observed. At low kappa a, the long-range electrostatic repulsion dominates the system behavior, stabilizing the colloidal system. In conditions of screened electrostatic potential, particle collision is the result of a competition between van der Waals attraction and steric repulsion, leading to a decrease in the dimer formation constant as the range or strength of the steric interaction increases. The steric interaction energy has been included in the theoretical calculation of the aggregation rate constant by means of Fuchs' stability factor. Good agreement between simulation, experimental, and theoretical calculations was obtained.