This study describes a sample preparation technique used to isolate dissolved organic carbon (DOC) in groundwater for radiocarbon analysis using accelerator mass spectrometry (AMS). The goal of the work is to improve our ability to determine groundwater residence times based on 14C measurements of the DOC fraction in groundwater. Water samples were collected from carbonate and volcanic rock aquifers in southern Nevada. Multiple measurements of total dissolved organic carbon (TDOC) in groundwater from one site are used to demonstrate the reproducibility of the analytical procedure. The reproducibility of the method is about one percent (1σ) for a 0.5 mg sample. The procedural blank for the same size sample contains about 1 percent modern carbon (pMC).