The effect of pH on the adsorption of catalase and lysozyme at the air-water interface has been studied using a combined surface pressure-interfacial shear rheology technique. The results presented show that the rate of development of interfacial phenomena increases as the pH of the subphase approaches the isoelectric point of the protein under investigation. The development of the measured interfacial rheological parameters is due to an increased rate of cross-link formation within the resultant interfacial gel. The formation of the interfacial gels has been modeled using a combination of the Smoluchowski theory for the coagulation of an aerosol or fog and classic rubber elasticity theory. The enhanced rate of cross-link formation at the isoelectric point is a result of an in-surface phase separation brought about by cooperative deionization of the protein molecules near their isoelectric point. Simultaneous measurements of surface pressure and interfacial rheology have enabled us to show that the development of a gel-like interfacial network coincides with observed increases in surface pressure.