Alteration of the chemical structure of proteins can be caused by deteriorative reactions such as Maillard reaction and by modification with chemical reagents intentionally done to alter the properties of the proteins. Acetylated soybean protein isolates (0\%, 25\%, 35\%, 55\%, 80\%) were incubated with glucose to induce Maillard reaction. Acetylation improved water sorption, fat binding, foam formation, and emulsion activity of the protein, but depressed browning and trypsin digestion. Storage caused Maillard reaction to occur, and such functionalities as water sorption, foam formation, and trypsin digestion were deterirated, but fat adsorption and emulsion activity were improved. Thus acetylation prevented deterioration of certain functional characteristics that occurred during storage, besides causing functional characteristics to be improved on its own.