The effect of quaternary ammonium ions on the proton transfer in the PCET reaction of ascorbate with hexacyanoferrate(III), in the presence of 0.5 mol dm-3 tetrapropylammonium chloride (TPrACl) in water, has been studied in this diploma thesis. Essentially, the effect of the addition of TPrACl on the kinetic isotope effect (KIE) and the temperature dependence of the KIE has been investigated. The Arrhenius parameters (preexponential factor A, activation energy Ea) and Eyring activation parameters (Gibbs activation energy ΔG‡, activation enthalpy ΔH‡, activation entropy ΔS‡) have been subsequently determined. Also, parameters indicating proton tunneling, the ratio of the Arrhenius preexponential factors (AH/AD) and the activation energy difference, ((ΔEa(D-H))) have been calculated from the obtained data. Activation parameters and kinetic isotope effects were obtained by UV-Vis spectrophotometric measurement of the rate constants for the reaction of ascorbate with hexacyanoferrate(III) in the presence of 0.5 mol dm-3 TPrACl in water and heavy water, at different temperatures ranging from 9°C to 40°C. The determined KIE in the presence of 0.5 mol dm-3 TPrACl in water has shown an increase from 4.6 to 10.6 at 25°C compared to the reaction in pure water. Both KIE values lie within the limits (≤ 13.0) predicted by the semiclassical approach. The determined AH/AD = 0.12 for the reaction in the presence of 0.5 mol dm-3 TPrACl in water has shown a decrease compared to the AH/AD = 0.97 for the reaction in pure water and is well beyond the limits (0.5 – 1.4) predicted by the semiclassical approach. This can indicate the occurrence of proton tunneling. The determined activation energy difference ΔEa(D-H) = 11.1 kJmol-1 for the reaction in the presence of 0.5 mol dm-3 TPrACl in water is greater than ΔEa(D-H) = 3.9 kJmol-1 for the reaction in pure water and deviates significantly from the maximum semiclassic value (5.1 kJmol-1), indicating again an occurrence of proton tunneling. The addition of TPrACl into the reaction mixture caused the change from an over the barrier reaction regime to a tunneling regime in the reaction. The results can be explained within the framework of the Marcus like tunneling model, which takes into account the impact of promoting modes of water environment, ie. the impact of compressive mode (fluctuations in the distance between the donor and acceptor, coupled with the reaction coordinate), and was developed in the context of the study of enzymatic reactions involving hydrogen tunneling and was later applied to the reaction in solution. The obtained data build on previous research (Jakobušić Brala et al., 2012; Jakobušić Brala et al., 2011) and should provide additional insight on the impact of quaternary ions on the investigated ascorbate PCET reaction and also similar PCET reactions.