The effect of sulfamic acid and urea treatments on the flame retardant properties of soybean fabric has been investigated by Limiting Oxygen Index (L.O.I.), Thermogravimetric Analysis (TGA), and X-ray Photoelectron Spectroscopy (XPS) in order to characterize both the bulk and surface properties. The sulfamic acid finish significantly reduced the soybean fabric flammability and appeared to function in the solid phase with increased char formation. Incorporation of urea into the formulation increased the level of reaction and associated weight gain. However, the wash durability of the sulfamic acid modification of the soybean fabric was limited, but after treatment with a cationic fixing agent, Matexil FC-ER significantly increased the wash fastness and related flame retardant effect. XPS analysis demonstrated the substantivity of the cationic fixing agent to soybean fiber surface and its binding effect on the sulfamate at the fiber surface. (c) 2008 Wiley Periodicals, Inc.