The reactivity of Tl1.4Ba2CaCu2O7.0, a high-temperature superconductor (HTSC), toward a redox-active adsorbate molecule with thiol functionality has been investigated as a function of solvent type and H2O content. Previously it had been reported that alkanethiols do not stably chemisorb onto the surface of Tl2Ba2CaCu2O8/Tl2Ba2Ca2Cu3O10 phases, bringing into question the generality of alkanethiol adsorption onto cuprate-based HTSCs. Herein, we show that under rigorously dry conditions, 11-mercaptoundecanoylferrocene (1) adsorbs cleanly onto the surface of Tl1.4Ba2CaCu2O7.0 to form moderately stable, redox-active monolayer films. The redox activity of 1 was used to estimate adsorbate surface coverage, and it is shown that the addition of H2O to the modifying solutions of 1 adversely affects the monolayer adsorption process. No redox activity associated with 1 could be detected for electrodes soaked in 0.1 M solutions of 1 with 5% H2O in ethanol, which was attributed to the formation of a BaCO3 corrosion layer that prohibited the adsorption of 1 onto the HTSC surface. The importance of sample stoichiometry on the adsorption process also is discussed.