The concept of the time-of-flight (TOF) method for the determination of ionic carriers’ mobility is presented. This method is used for the model system, the properties of which (including viscosity) change with time : the isothermally cured epoxy-amine system. It is demonstrated that the form of the TOF current signals depend on the material of the electrodes. The Cu, Ag, Al, and Au electrodes were tested; of these, the Cu and Au electrodes have yielded the most distinct TOF signals and therefore were chosen for further experiments. The investigated samples were placed between parallel electrodes with guard rings distanced with polyethylene terephthalate (PET) films of different thickness (0.02-0.2 mm). According to the predictions, it was found that the TOF of the ionic carriers increases with an increase of the distance between the electrodes (for constant electric field) and decreases with an increase of the electric field (for constant distance). The series of measurements performed for the well-known epoxy resin system diglycidylether of bisphenol A with diamino-4,4’-dimethyl-3,3’-dimethyldicyclohexylmethane, isothermally cured at different temperatures, have confirmed that in a course of the curing, below the gelation, the conductivity decreases following the decrease of the ionic mobility due to the viscosity increase during the reaction. However, with an advancement of reaction, deviations from that linear relationship are observed.