Two novel nitrophenyl disulfide esters, di(4-nitrophenyl)-4,4'-dithiobisbenzoate (pNBD) and di(4-nitrophenyl)-3,3'-dithiobisbenzoate (mNBD), were prepared, adsorbed spontaneously as thiolate monolayers on gold surfaces, and used as models for examining factors affecting rates of interfacial reactions. These monolayers were characterized by infrared reflection spectroscopy (IRS), X-ray photoelectron spectroscopy, single sweep voltammetry, and contact angle goniometry. The adsorbed form of mNBD (mNBT) undergoes a pseudo-first-order base-catalyzed hydrolysis at a rate that is 7 x 10(4) slower than that for mNBD in solution. The monolayer of the adsorbed para isomer of the ester is even less reactive; that is, it is not detectably hydrolyzed after 3 days of immersion in 0.5 M KOH. The lower packing density of the meta isomer ester accounts for its faster rate of hydrolysis as compared with that of the para isomer ester. The progression of the IRS data suggests that the orientation of the nitrophenyl ring in the mNBT monolayer undergoes a significant change during the early stages of hydrolysis but that this change in orientation does not detectably affect the rate of hydrolysis. Reaction of the mNBT monolayer with n-butylamine, which forms the corresponding amide via pseudo-first-order kinetics, does not cause a change in the orientation of the nitrophenyl ring. Thus, the change in orientation during hydrolysis reflects an increase in the free volume of the adlayer that arises from loss of the nitrophenylate product to solution.