Interfacial activation of pig pancreatic phospholipase A2 (PLA2) in the steady-state phase, which follows a lag period in the reaction progress at the air/water interface, is shown to be due to the accumulation of the products of hydrolysis. The kinetic and equilibrium results clearly show that the products accumulated during the lag leads to a higher apparent rate of hydrolysis of didecanoylphosphatidylcholine (PC10), and the lag virtually disappears if the products are already present. Our results invalidate the assumption that all the products formed at the air-water interface rapidly leave the monolayer. PC10 forms laterally compressible insoluble monolayer, whereas the products of hydrolysis do not form such a stable monolayer. Surprisingly, however, a significant amount of the product is retained in the PC10 monolayer at equilibrium, as well as under the steady-state condition of the reaction progress. The excess product, formed in situ or cospread with PC10, leaves the monolayer slowly with a half-time of about 100 s. These results reaffirm that the microscopic steady-state condition for the interfacial catalytic turnover is controlled by factors that determine the local concentrations of the reactants and effecters that the enzyme "sees" during the reaction progress. Our findings call in question all interpretations that attribute the pre-steady-state delay to the surface pressure-dependent intrinsic "penetrating power" of PLA2. As a general, unified, and consistent basis for the interpretation of interfacial kinetics, we conclude that irrespective of the nature of the interface the intrinsic rate of binding to the interface is rapid, and the enzyme binding to the interface depends on the magnitude of the interfacial anionic charge.