The surface phase behavior of lysozyme is studied, first by studying its adsorption at aqueous-gas interfaces and then by studying its penetration into an insoluble monolayer of dipalmitoylphosphatidylcholine (DPPC). When lysozyme adsorbs on an aqueous-gas interface;the surface tension remains constant over an extended period of time before reducing. This induction period indicates a first-order transition at the interface from a surface gaseous phase to a liquid-expanded phase. The coexistence of these phases is demonstrated by fluorescence imaging. The surface tension evolution is compared favorably to theoretical traces predicted for the dynamic adsorption of a soluble amphiphile which-undergoes a surface phase change. Lysozyme is an ellipsoidal molecule which can adsorb in either side-on or end-on configurations. The transition from side-on to end-on adsorption is shown to coincide with the phase change. The time scales for the adsorption process are in agreement with a diffusion-controlled mechanism at dilute concentrations but are far longer at elevated concentrations, indicating the presence of a strong kinetic barrier to adsorption. Long time surface tension data as a function of bulk concentration C-infinity show no reduction for C-infinity corresponding to (side-on) gaseous surface states, strong reduction at C-infinity corresponding to (end-on) liquid-expanded surface states. In the penetration experiments; an insoluble monolayer of DPPC is initially present. The surface pressure rise upon exposing the monolayer to lysozyme solution has no induction period. This is explained in terms of the lipid screening cohesive interactions between the adsorbed lysozyme molecules, eliminating the phase change. Finally, Brewster angle microscopy (BAM) images and compression isotherms for the two-component system are discussed in terms of intermolecular interactions.