The chemistry and electrochemistry of TCNQ (7,7,8,8-tetracyanoquinodimethane), TCNQ(center dot-),TCNC2-, and H(2)TCNQ in acetonitrile (0.1 M Bu4NPF6) solution containing trifluoroacetic acid (TFA) has been studied by transient and steady-state voltammetric methods with the interrelationship between the redox and the acid-base chemistry being supported by simulations of the cyclic voltammograms. In the absence of acid, TCNQ and its anions undergo two electrochemically and chemically reversible one-electron processes. However, in the presence of TFA, the voltammetry is considerably more complex. The TCNC2- dianion is protonated to form HTCNQ(-), which is oxidized to HTCNO center dot, and H(2)TCNQ which is electroinactive over the potential range of -1.0 to +1.0 V versus Ag/Ag+. The monoreduced TCNQ(center dot-) radical anion is weakly protonated to give HTCNQ(center dot), which disproportionates to TCNQ and H(2)TCNQ. In acetonitrile, H(2)TCNQ deprotonates slowly, whereas in N,N-dimethylformamide or tetrahydrofuran, rapid deprotonation occurs to yield HTCNQ(-) as the major species. H(2)TCNQis fully deprotonated to the TCNC2- dianion in the presence of an excess concentration of the weak base, CH3COOLi. Differences in the redox and acid-base chemistry relative to the fluorinated derivative TCNQF(4) are discussed in terms of structural and electronic factors.