Interfacial enzyme reactions are ubiquitous both in vivo and in technical applications, but analysis of their kinetics remains controversial. In particular, it is unclear whether conventional Michaelis–Menten theory, which requires a large excess of substrate, can be applied. Here, an extensive experimental study of the enzymatic hydrolysis of insoluble cellulose indeed showed that the conventional approach had a limited applicability. Instead we argue that, unlike bulk reactions, interfacial enzyme catalysis may reach a steady-state condition in the opposite experimental limit, where the concentration of enzyme far exceeded the molar concentration of accessible surface sites. Under this condition, an “inverse Michaelis–Menten equation”, where the roles of enzyme and substrate had been swapped, proved to be readily applicable. We suggest that this inverted approach provides a general tool for kinetic analyses of interfacial enzyme reactions and that its analogy to established theory provides a bridge to t...