Extruded products are usually studied with the respect to their mechanical and sensorial properties. Computer image analysis has been used to provide information on internal structure of a variety of products. In the current study a digital image technique was used to determine structural properties of two types of extruded flour products as screw speed changed. The extrusion cooking of corn and wheat flour was carried out in a twin-screw corotating extruder. Sectional and longitudinal expansion were measured and water solubility and absorption were also determined. The increase in screw speed resulted in products possessing greater longitudinal expansion, higher water solubility and smaller structural patterns. However, differences between the corn and wheat extrudates were also evident at the various screw speeds examined. (C) 2002 Elsevier Science Ltd. All rights reserved.