Starches from WT, lam, and r pea mutants differing in amylopectin/amylose contents (70, 90, and 28% amylopectin, respectively) were used in kinetic studies of pancreatic alpha-amylase action at 37 degrees C and for investigations of their supra:molecular structure and physicochemical properties during heating. For WT and lam starches, amylase accessibility and catalytic efficiencies (CE) increased following hydrothermal processing up to 100 degrees C. Accessibility changed relatively less in r during heating with increasing K-m between 60-90 degrees C. Limiting values of K-m after gelatinization were very similar for all three mutants, indicating that relative proportions of amylose/amylopectin have little influence on amylase accessibility once ordered structures are lost. For WT and lam, increases in enzyme accessibility and CE paralleled a rise in amorphous content. It is suggested that the complex behavior for r resulted from amylose gel formation between 60-90 degrees C. Amorphous amylopectin seems a better substrate for amylase than amorphous amylose.