The interactions between As and dissolved organic matter (DOM) have significant impacts on the environmental behavior of As. Among various interactions, the binary complexes of As bound to DOM and ternary complexes of As bound to DOM with iron (Fe) bridging, are critical to mobility, speciation, toxicity and bioavailability of As. Flow field flow fractionation (FlFFF) offline with high-resolution inductively coupled plasma mass spectroscopy (HR-ICPMS) were utilized to elucidate the As, Fe and DOM complexation in high arsenic, reducing groundwater from Yinchuan plain. The majority (>70%) of As was truly dissolved with molecular size <0.5 nm; while 13% of As and 37% of Fe were bound to colloidal DOM with molecular size between 16–64 nm. The results imply that As-Fe-DOM complex is distributed as moderately large colloids with molecular size between 16–64 nm. However, artefacts due to changes occurring during sample storage and transportation cannot be ruled out. Thus, field ultrafiltration under N2 atmosphere will be employed in the future to validate FIFFF results.