A suboxic groundwater from a sandy coastal aquifer was sampled using a new air free, large volume sampling method. Subsequent processing for size fractionation was completed with a modified cross-flow ultrafiltration (CFF) system equipped with a 1 kDa CFF membrane. By purging the CFF system with nitrogen, no oxygen was able to reach the sample. With this optimization, the sample was processed with higher than 90% recovery in terms of both iron and phosphate. Only about 4% of iron and 20% of phosphate in the filtered (0.2 microm) groundwater sample was found to be in colloidal form in the groundwater. In contrast, if no care was taken to maintain the suboxic environment of the original sample, iron was rapidly and completely oxidized and subsequently adsorbed to the CFF membrane. Other elements, such as phosphorus, were also lost to the CFF membrane to a substantial degree, and the mechanism is most likely coprecipitation with iron oxides. This study thus strongly supports the importance of maintaining ambient redox conditions during sampling and fractionation, especially for the determinations of colloid abundances in groundwater.