The effects of hydrolysis and oxidation on the heat-induced structural changes of barley starch dispersions were studied by dynamic viscoelastic methods and light microscopy. The effects of the modifications on the molecular weight of amylose and amylopectin were investigated by high-performance liquid chromatography (HPLC) with post-column iodine staining. Comparison of the molecular weights of oxidized and acid-hydrolysed barley starch dispersions with the same limiting viscosity values showed that the molecular weight of amylose was decreased more by oxidation than by hydrolysis, whereas the molecular weight of amylopectin was reduced to the same degree. Relative to acid hydrolysis, oxidation also reduced the temperature at which gel-formation occurred and weakened the gels. Irrespective of the type of modification, large amounts of granule residues were left after heating at 120 °C for 20 min. Microstructural studies of the gels showed that both hydrolysed and oxidized starches underwent phase separation into amylose- and amylopectin-rich phases.