Starch-water interactions during gelatinization by microwave heating was investigated by Low-field H-1 Nuclear Magnetic Resonance (LF NMR) relaxometry experiments. Effect of microwave heating time on longitudinal (T-1) and transverse relaxation times (T-2), gel matrix structures, changes in proton populations and firmness values were evaluated. One to one corn starch (CS) to water ratio samples had the lowest T-2 change during gelatinization indicating lower granule expansion which was attributed to the high amylose content of CS. Non-Negative-Least-Square (NNLS) analysis of the transverse relaxation curves provided detailed information on starch-water interactions during gelatinization. Rice starch (RS) samples had relatively lower changes in peak areas which was related to the better hydration of RS granules.,While microscope images revealed the changes in the granule structures after distinct gelatinization intervals, firmness measurements supported the structural changes. With increasing microwave heating time, both T-1 and T-2 showed a decreasing trend. Two distinct proton populations detected on relaxation spectrum indicated the exchanging protons during gelatinization. A noticeable reduction in firmness at the later moments of microwave heating was detected. The results showed that starch gelatinization by microwave heating had some distinct features in terms of mechanism of the gelatinization and could be investigated by NMR relaxometry. (C) 2017 Elsevier Ltd. All rights reserved.