The diffusion coefficient of lysozyme, a globular protein, was measured at various conditions as funcitions of lysozyme concentration, salt concentration, and solution ’age’ in concentrated, saturated, and supersaturated solutions, employing Gouy interferometry. Distilled water, 0.05M potassium phosphate buffer, and 0.1M sodium acetate buffer solutions with 0,2,4, and 5wt% NaCl were used as solvents. The pH of lysozyme solutions in distilled water was 4.75 due to the self-buffering capacity of lysozyme. The pH’s of the lysozyme solutions in the potassium phosphate and so-dium acetate buffers were adjusted to 6.8 and 4.0, respectively. The experimental temperature was 25℃. In a salt-free sys-tem, the concentration dependent diffusion of lysozyme showed typical electrolyte diffusion behavior, while a salt-po-lyelectrolyte system exhibited the behavior of a non-electrolyte. Diffusion results in the supersaturated region showed a lit-tle effect of concentration or solution ’age’ at a fixed NaCl concentration. A rapid decline in diffusion coefficient with in-creasing NaCl concentration in the supersaturated region, however, was observed.