The amylase digestibility of high-amylose maize starches has been compared before and after thermomechanical processing. Starches were analysed for enzyme-resistant starch yield, apparent amylose content, crystallinity (X-ray diffraction), and molecular order (NMR and FTIR), both before and after treatment with a-amylase. All samples had significant (>10%) enzyme-resistant starch levels irrespective of the type and extent of thermal or enzymic processing. Molecular or crystalline order was not a pre-requisite for enzyme resistance. Near-amorphous forms of high amylose maize starches are likely to undergo recrystallisation during the enzyme-digestion process. The mechanism of enzyme resistance of granular high-amylose starches is found to be qualitatively different to that for processed high-amylose starches. For all samples, measured levels of enzyme resistance are due to the interruption of a slow digestion process, rather than the presence of completely indigestible material. 2008 Elsevier Ltd. All rights reserved.