Structural and topographical characteristics of two major fractions of soy globulin from a soy protein isolate, beta-conglycinin (a 7S globulin) and glycinin (a 11S globulin), including the effect of chemical reduction of glycinin with dithiothreitol (DTT), spread at the air-water interface at 20 degreesC and as a function of pH were determined from pi-A isotherms coupled with Brewster angle microscopy. The structural characteristics of 7S and 11S globulin spread monolayers depend on film aging. We have observed a significant shift of the pi-A isotherms toward higher molecular areas over time. The aging effect was due to unfolding of the protein at the interface. The monolayer structure was more expanded on the aqueous subphase at pH 2.0, and the opposite was observed at pH 5.0. The chemical reduction of glycinin with DTT produced a significant expansion of the monolayer structure. A change in the monolayer structure was observed at a surface pressure of 13.5-16.5 mN/m. At a microscopic level, the heterogeneous monolayer structures visualized at pH 5.0 or near to the monolayer collapse proved the existence of large regions of protein aggregates. Relative reflectivity increases with surface pressure and was a maximum at the nionolayer collapse, with the maximum reflectivity for glycinin at every pH.