Natural organic matter (NOM) is a universally important component in environments. Our knowledge of the origin, nature, reactivity, and fate of NOM is limited due to its extreme molecular complexity and variability across ecosystems, for instance soils, aerosols, sediments, and waters. Recent advances in analytical techniques become widely employed in the field of environmental chemistry and geochemistry. Cases of improved characterization of NOM at molecular level will be presented, mainly showing the application of the coupled reverse osmosis-electrodialysis technique, Fourier transform ion cyclotron resonance (FTICR) ultra-high resolution mass spectrometry, and nuclear magnetic resonance (NMR) spectroscopy. Collectively, these techniques provide elemental composition and structural information of NOM, which are fundamental to tracing changes in organic compounds during environmental processes, such as photochemical redox reactions. I will elaborate some of the new insights learned over the last few years, showing that a significant portion of terrestrial dissolved NOM been molecularly rearranged, potentially by reactive oxygen species during photochemical decomposition, to form molecularly unrecognizable organic molecules such as humic substances. The ongoing study aims to provide transformative understanding of the pathway in which the large pool of terrestrial NOM becomes disseminated among the large carbon reservoirs, and to improve the general understanding of how carbon, nitrogen, and iron are cycled on both short and long time scales.