Measurements of disinfection byproduct (DBP) organic precursor concentrations (OPCs) are crucial to assess and improve DBP control processes. Typically, formation potential tests - specified in Standard Methods (SM) 5710-B/D - are used to measure OPCs. Here, we highlight several limitations of this protocol for dihaloacetonitriles and trihalomethanes and validate a novel Alternative Method (AM). The effects of pH, disinfectant type (free chlorine and monochloramine), and chlor(am)ine residual (CR) were examined on DBP formation in a suite of waters. Using the SM, DHAN decreased 43-47% as the CR increased from 3 to 5 mg L(-1) as Cl2, compromising OPC assessments. In contrast, a high monochloramine dose (250 mg L(-1) as Cl2) at pH 7.0 (the AM) accurately reflected OPCs. The two methods were compared for assessing DBP precursor removal through three granular activated carbon (GAC) columns in series. Breakthrough profiles assessed using the AM only showed DBP precursor sorption occurred in each column that decreased over time (p = 0.0001). Similarly, the AM facilitated ranking of three types of GAC compared in parallel columns, whereas the SM produced ambiguous results. Fluorescence intensity of a humic-like fluorophore (i.e., I345/425) correlated strongly to precursor removal in the GAC columns. The practical implications of the results are discussed.