A positively charged monolayer of a viologen derivative containing two long alkyl chains was prepared. Tetracyanoquinodimethane radical anions (TCNQ •Ϫ ) were incorporated into the Langmuir-Blodgett film during the transference process as the counterions from a subphase containing an aqueous solution of LiTCNQ. Stable monolayers were transferred onto indium tin oxide electrodes and the electrochemical properties of the films were analyzed in terms of experimental variables including the nature of the counterions, the degree of order of the films, and the number of transferred layers. These well-ordered films show very reversible electrochemical processes for the viologen, quite negative redox potentials of the viologen moieties, a ⌫ c /⌫ a ratio of 1, and no splitting of the first viologen reduction peak. The control of these properties is quite important in applications such as the catalytic reduction of a substrate.The electrochemical properties of immobilized redox molecules at electrode surfaces have focused the interest of many researchers over the last decade. Among the techniques used to achieve a welldefined molecular arrangement of redox moieties onto electrode surfaces, the Langmuir-Blodgett ͑LM͒ method has been one of the most widely used because it allows creation of films with a high internal order, with the desired number of layers, the possibility of depositing alternate monolayers of different molecules, etc. Thus, the LB technique can provide us new insights on the electron-transfer reactions at interfaces, as well as the possibility of obtaining highly ordered modified electrodes with practical applications such as molecular electronics, electrocatalysis, synthesis, and electroanalysis. 1-3 1,1Ј-Disubstituted 4,4Ј-bipyridyls, so-called viologens, are of great electrochemical interest because of their three oxidation states and the high reversibility of their redox reactions ͑especially the first one͒; 4 their use in solar energy storage devices and as electron mediators in herbicides; 5 preparation of chemically modified electrodes; 1 catalytic reduction of a substrate; 6 studies of the rates of electron transfer at electrode substrate-film interface; 7 their potential electrochromic applications; 8 their use in electrochromic writing paper; 9 bioelectronic devices; 10 electron-transfer mediation to various biological molecules; 11 and surface-enhanced Raman studies of adsorption at electrode surfaces, 12 etc.Several research groups have investigated the electrochemical properties of LB films incorporating both different kinds of viologen dicationic derivatives and different kinds of anions. These studies have tried to work out how the number and length of the alkyl chains, as well as the nature of the anion, determine the electrochemical properties of the modified electrodes. 10,[13][14][15][16][17][18][19][20][21][22] In this paper we focus our attention on three important points in the understanding of the viologen's electrochemical behavior: (i) the splitting of the first reduction peak; (ii...