The post-translational modification of proteins is controlled by the relative activities of two opposing enzymes. For example, the extent of phosphorylation of tyrosine residues reflects the balance of a kinase and a phosphatase enzyme. The present article uses as a model system a self-assembled monolayer that presents a peptide that can be phosphorylated by Abl kinase and subsequently dephosphorylated by Lambda phosphatase. Treatment of monolayers with a reaction mixture containing both enzymes reveals that the steady-state level of peptide phosphorylation is dependent on the density of the peptide. Using identical reaction mixtures, surfaces that presented the substrate at high density led to a phosphorylated peptide at steady-state, whereas surfaces that presented the substrate at low density led to unphosphorylated peptide at steady-state. This dependence owes to an autocatalytic phosphorylation reaction that operates at high densities of substrate. This work provides an example of an interfacial reaction that has properties that have no analogue in the corresponding solution phase reaction. It also provides a model system that is relevant to understanding mechanisms that regulate signaling at the cellular membrane.