Mixtures of soybean and sweet potato (Ipomoea batatas L. Lam) flour, containing 18% moisture on dry matter were extruded in a single screw extruder: The feed supply consisted of different sweet potato/soybean ratios. The variable process conditions used were screw speed (rpm) and die diameter Cold paste, hot end viscosity and degree of gelatinisation (starch damage) were determined. Quadratic response surfaces were fitted to these responses. At small die diameters an increase in both screw speed and percentage of sweet potato raised cold paste viscosity, whereas at large die diameters a reduction was found for hot end viscosity. A quadratic effect of sweet potato was found, with a minimum at +/-76%. At small die diameters an increase in the percentage of sweet potato raised the degree of gelatinisation, whereas an opposite effect was found at large die diameters.