In this paper a theoretical investigation of electrostatic-based pesticide spray systems is carried out. Owing to the complexity of the problem, the system was divided into two different regions, one inside the nozzle, and the other between the nozzle and the target. Two mathematical models for the two regions have been developed and a charge simulation method (CSM) is used. Inside the nozzle particular emphasis is laid on the effect of the voltage applied to an electrostatic induction spray nozzle on the droplet's charge, mobility and charge-to-mass ratio. The variation of these parameters along the jet is also predicted by the model. In addition, the model takes into consideration the effect of applied voltage on both spray current and charge density at the nozzle. Results show that, inside the nozzle, the volume charge density of the droplets, the spray current and the droplet's charge and charge-to-mass ratio do increase with applied voltage. In the region between nozzle and target, the analysis focuses on studying the spatial distribution of the droplet's charge density, travel time and trajectory. In this model, the charge density, previously calculated at the nozzle, is taken as a variable parameter. A CSM approach is combined with the method of characteristics to solve Poisson and current density equations subject to the validity of the current continuity condition. In this region, the model clearly indicates that the droplet's trajectory depends on the spatial distribution of the droplet charge density and the air flow velocity. The travel time between the nozzle and the target is also calculated. Curves illustrating the results are shown and explanations provided.