Although no protons are involved in the electrochemical reactions of viologens, pH-dependent redox potentials are measured for viologens with protonatable functional groups at the end of one of the lateral chains. The potential shift with pH is observed for short-chain substituents (two or three C atoms) but not for long-chain substituents (six or ten C atoms). This is due to the inductive withdrawing effect of the functional group on the reduction potential of the bipyridinium ring. A good correlation is obtained between experimental data and theoretical equations. The pK(a) values for oxidized and reduced species of viologens were calculated by numerical computation.