Organic Electrochemical transistors (OECTs) present unique features for their strategic combination with biomedical interfaces, simple and low voltage operation regime and sensing ability in aqueous environment, but they still lack selectivity, so that a significant effort in research is devoted to overcome this limitation. Here, we focus on the diffusion properties of molecular species in the electrolyte, which opportunely analyzed, modeled and compared to experimental data, serve as a simple and direct key factor in the recognition of species during OECT sensing. Specifically, we model the transient behavior of an OECT considering the effect of diffusion of the target species in the electrolyte. In doing so, we develop a general method that can be used to differentiate and distinguish different molecules from a complex mixture, on the basis of their diffusivity and thus mass. More importantly, the model can be realistically used to determine the physical characteristics of the transported species in a solution from a simple fitting procedure. On the basis of the obtained results, we discuss the contribution that our study could bring to OECT architecture to realize a new generation of devices with improved sensitivity, selectivity and reliability.