This paper describes the use of a substituent effects study to understand the mechanistic basis for an interfacial Diels-Alder reaction that does not proceed with standard second-order kinetics. Cyclopentadiene (Cp) undergoes a Diels-Alder reaction with a chemisorbed mercaptobenzoquinone to yield an immobilized Diels-Alder adduct. The pseudo-first-order rate constants are not linearly related to the concentration of diene, but they reach a limiting value with increasing concentrations of diene. The results of a substituent effects study support a mechanism wherein the electrochemical oxidation of hydroquinone produces two states of quinone. The first form, Q*, either reacts with Cp or isomerizes to Q, a form that is significantly less reactive with the diene. The interfacial reaction reaches a maximum rate when the concentration of diene is sufficiently high so that Q* undergoes complete Diels-Alder reaction and does not isomerize to Q. This work provides an example of the use of physical organic chemistry to understand an interfacial reaction.