Plant proteins have been extensively studied as promising bioresources for sustainable materials applications, because of their abundance in nature and remarkable tunability in structures and properties, in face of the rising environmental and socioeconomic concerns of petrochemicals. A tremendous number of studies have explored their potential applications in packaging, composites, and biomedical applications, etc... However, unlike synthetic polymers and another popular biopolymer from natural resources, i.e., cellulose, proteins have complex structures and diverse functional groups, and they exhibited sophisticated, but insufficiently understood, interactions with chemical environment, which are vital to their aggregated structures, and eventually applications. To successfully apply proteins to practical materials applications, a better understanding of protein structures and behaviors, subjected to chemical environment, is needed. Among various plant proteins, soy protein has received the greatest attention, due to its richness in nature as the product of the major economic crop, soybeans. This project focused on soy protein isolate (SPI), and its interactions with dimethyl sulfoxide (DMSO), one of the most popular organic solvents in materials and biological research. Pure SPI exhibited 3 relaxation processes, α, β, γ relaxations, related to segmental chain motions, conductivity relaxation, and the fast local motions of protein chain subunits, respectively. The unique interactions between sulfoxide groups in DMSO and amine groups in SPI apparently had an enhancing effect on both β and γ relaxations, besides the plasticizing effects on all three relaxations. The absorbed moisture introduced a new relaxation process between β and α relaxations. With increasing temperature, the coupling of β and γ relaxations with α relaxation became prominent, leading to apparent merge with α relaxation.