We have studied the structural and surface dilatational characteristics (surface dilatational modulus and its elastic and viscous components) 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 at pH 2.0, 5.0, and 8.0. The stress response to compression-expansion sinusoidal deformation of the interface in a modified Wilhelmy-type trough with two oscillating barriers was measured at a constant amplitude (5% of the initial area) and as a function of frequency (within the range of 1-100 mHz), and superficial pressure. The same experimental device coupled with Brewster angle microscopy makes it possible to determine the structure, morphology, and relative reflectivity of the monolayer. The monolayer structure was more expanded on an 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. The monolayer structure determines the surface dilatational characteristics of soy protein films. It was found that not only is the dilatational modulus determined by the interactions between spread molecules (which depend on the surface pressure) but also the structure of the proteins spread on the monolayer plays an important role.