Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, is a polymeric mixed conductor used in the vast majority of devices in bioelectronics, electrochromics, energy storage/generation, neuromorphic computing, and thermoelectrics. These devices operate at the interface with electrolytes and rely on the uptake of mobile ions by the film, making the coupling between electronic and ionic charges crucial. For efficient transduction of ionic charges into electronic ones, all the ions injected into the film should lead to a change in conductivity. Although extensively studied, fundamental knowledge regarding the losses during this process is lacking. In this study, we quantify the efficiency of ion-to-electron coupling in PEDOT:PSS films by measuring the number of cations taken up by the film as well as the in situ current generated as a result of their interactions with the electrically active sites. We find that not all the injected cations are used for reducing PEDOT oligomers in thick films and some of these ions remain in the film upon de-doping. The efficiency of ion-to-electron transduction thus varies with thickness, a parameter which critically affects the distribution of PEDOT and PSS in the bulk as revealed by Raman spectroscopy, X-ray photoelectron spectroscopy and scanning transmission electron microscopy studies. Tracking the traces of ions, we provide guidelines on how to maximize the coupling between ionic and electronic charges for high performance transducers. Our approach is thus fundamental to future development and optimization of mixed conductors applied at the interface with electrolytes.