The ubiquitously present dissolved organic matter (DOM) greatly influence the efficiency of UV-based technologies due to its reactivity to UV irradiation. In this work, UV-induced changes within three hydrophobic DOM fractions isolated from different surface waters were investigated. Analysis on UV absorbance at 254 nm, electron donating capacity, fluorescence intensity and carbon content revealed small changes in DOM bulk properties associated with the UV-induced photochemical reactions. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was further used to explore the modification of the molecular distribution based on H/C and O/C ratios, m/z and DBE. The molecular-level investigation revealed that an average of 296 aromatic and lignin-like molecules were degraded, leading to the production of around 306 new molecules. The UV-reactive community were identified as CHO molecules with higher DBE (>10) and carbon number (>25) which could be readily transformed into smaller saturated molecules. Molecules containing nitrogen (N) or sulfur (S) atom, independent of aromaticity and molecular weight (m/z), were also highly UV susceptible and transformed into molecules with larger DBE and m/z. Possible reaction pathways responsible for the observations were discussed. The results indicated that UV-reactivity and subsequent transformation of DOM are remarkably correlated with its molecular composition and characteristics. Though the changes in bulk properties of DOM following UV irradiation were observed to be very small, the significant alteration in its molecular structures would have a profound impact on the UV-based treatment processes.