Quenching agents (QAs) are widely used in order to prevent the additional formation of disinfection by-products (DBPs) during the sample holding time. In addition, DBP levels are usually stabilized by adjusting the pH of water samples. Previous studies have mostly focused on the individual effects of QAs or of pH on the stability of DBPs in water samples. Considering that disinfectant quenching and pH adjustments are applied simultaneously during routine analyses, it is more appropriate to evaluate the stability of DBPs with all the chemicals (e.g., disinfectants, QAs, buffers) present in the water. This study investigated the synergistic effects of different QAs (ascorbic acid, ammonium chloride, or no quenching) and pH adjustment (3-9) strategies on the stability of different classes of DBPs (i.e., trihalomethanes (THMs), halogenated acetaldehydes (HALs), haloacetonitriles (HANs), haloketones (HKs), and halonitromethane (HNM)). Sample preservation conditions that did not include a QA were shown to be inadequate for GC-ECD analysis, due to interference problems. Ammonium chloride was found to be effective for most DBPs. However, some HALs continued to form in the presence of chloramine, which is a by-product of dechlorination using ammonium chloride. Conversely, using ascorbic acid efficiently inactivated residual chlorine, providing a clean chromatographic baseline. Based on the results of this study, we recommend the use of ascorbic acid for quenching and sulfuric acid for acidifying (pH 3.5) samples. Considering the instability of some DBPs in water matrices over long periods (i.e., 14 days), samples should be processed as soon as possible after collection.