Whey proteins have restricted use in many food applications because of limited functional properties. Whey proteins' relatively high content of disulfide bonds may be responsible for their lack of functionality, especially in extrusion applications. To determine the effect of disulfide bond content on functional properties and extrudate performance, whey protein concentrate was treated with sodium sulfite to achieve four levels of disulfide bond sulfonation (0, 31, 54, and 71%). Sulfonated whey protein functional properties, extrusion-expanded snack properties (32% total protein), and extrusion-textured fibrous product properties (48% protein) were determined. Correlation analysis was performed to determine relationships between functional properties and extrudate performance. Sulfonation of whey protein concentrate (80% protein) increased foaming and emulsion properties and decreased melt temperatures. These changes were largely attributed to increased protein unfolding and flexibility. Sulfonation decreased gel strength and increased resolubilization after heat treatment. These changes were likely the result of increased electric charge on the proteins, limiting protein-protein interactions during heating. Snack products extruded from the 31 and 71% sulfonated samples were less expanded and released less protein and carbohydrate during extrudate solubilization. Sulfonation may have promoted protein unfolding, thereby exposing interaction sites and increasing the formation of insoluble protein-starch aggregates . In support of this suggestion, negative correlation s were found between extrusion performance and protein functional properties related to flexibility , including emulsification activity index, foam stability, and melt onset temperature. The anomalous behavior of the 54% sulfonated sample may be the result of significant structural and functional changes of a-Lb that are predicted to occur at approximately 50% sulfonation. Although the textured extrudate produced from all levels of sulfonation (including the control) did not possess typical fibrous texture, sulfonation at 31% and higher decreased stability after hydration . Decreased stability and fibrous texture may have resulted from decreased protein-protein interactions caused by the repulsion of electric charges contributed by sulfite groups. In conclusion, sulfonated whey protein functional and extrudate properties were influenced by disulfide bond content. Changes in these properties were attributed primarily to increased protein unfolding and flexibility. Increased electric charge on proteins also played a role where protein-protein interactions were important.