1,3- and 1,4-phenylenediamine were covalently attached to a self-assembled monolayer of 11-mercaptoundecanoic acid by formation of an amide bond. Bonding of the phenylenediamine isomers was confirmed by the presence of infrared features attributable to ring vibrational modes, the amide I vibration, and the amide II vibration in the reflection–absorption infrared spectrum of the modified interface. Reductivedesorption peak potentials were shifted by 120 mV for the monolayer modified by 1,4-phenylenediamine and by 90 mV for the monolayer modified by 1,3-phenylenediamine relative to the desorption potential of the unmodified 11-mercaptoundecanoic acid monolayer. These potential shifts represent 23 and 17 kJ/mol stabilization of the monolayers modified by 1,4- and 1,3-phenylenediamine, respectively. Some of the additional stabilization energy is attributed to lateral hydrogen bonding between the amide groups and unreacted carboxylic acid groups in the monolayer. The difference between the stabilization energies for the interfaces modified with the different phenylenediamine isomers is attributed to structural differences between the isomeric monolayers influencing the molecular packing of the modified interface.