Reduction of nitrobenzene derivatives in the presence of arylureas in aprotic solvents results in large positive shifts in potential of the nitrobenzene(o/-) cyclic voltammetry wave with little change in wave shape. This behavior is indicative of reversible hydrogen bonding between nitrobenzene radical anions and arylureas. Computer fitting of the cyclic voltammetry of 4-nitroaniline, NA, plus 1,3-diphenylurea in DMF shows essentially no binding between urea and NA in the oxidized state (K-ox < 1 M-1), but very strong binding in the reduced state (K-red = 8 x 10(4) M-1), along with very rapid rates of hydrogen bond formation (k(I)'s approximate to 10(8)-10(10) M-1 s(-1)), making this system a fast on/off redox switch.