The electrochemistry of TTF, TTF+˙, TTF2+ and HTTF+ (TTF = tetrathiafulvalene) has been studied in acetonitrile (0.1 M Bu4NPF6) solutions containing ethereal HBF4 or trifluoroacetic acid (TFA) using transient and steady-state voltammetric techniques. In the absence of acid, the oxidation of TTF occurs via two, diffusion controlled, chemically and electrochemically reversible, one-electron processes with reversible formal potentials of −74 and 311 mV vs. Fc0/+ (Fc = ferrocene). The voltammetry in the presence of acid is far more complex. Voltammetric and UV-vis data reveal that the parent TTF undergoes facile protonation to yield the structurally modified HTTF+ cation in the presence of acid. In contrast, detailed analysis of the data show that electrochemically generated TTF+˙ and TTF2+ do not react with acid. The voltammetry in the presence of acid has been simulated to provide a thermodynamic and kinetic description of the acid–base chemistry coupled to electron transfer.