Drinking water disinfection is one of the major public health triumphs of the last century. However, it has the unintended consequence of forming disinfection by-products (DBPs). Iodine containing DBPs have been recently reported to be more toxic than bromine containing DBPs, which are also much more toxic than chlorine containing DBPs. Seawater impacted groundwaters may have elevated levels of bromide and iodide, and if used to produce drinking water, high concentrations of bromo and iodo-DBPs may form. In this context, the present study aimed at investigating the formation of two chemical classes of DBPS, i.e., haloacetic acids (HAAs) and haloacetamides (HAcAms) in seawater impacted groundwaters under chlorination and chloramination. In order to do this, lab-scale disinfection reactions at pH 7 and 8 were carried out with two different groundwater matrices containing various amounts of sea water (0%, 0.5%, 1% and 2%). The DBP mixtures generated were chemically characterized in order to assess the occurrence of HAAs and HAcAms. HAAs and HAcAms are low molecular weight compounds, very polar and not very volatile, and therefore, they are amenable to liquid chromatography-mass spectrometry detection (LC-MS), although they are commonly measured by means of gas chromatography techniques. In this work, the development of LC-MS/MS based analytical methodologies was attempted for the analysis of these compounds in the generated DBP mixtures. LC-MS/MS conditions were successfully optimized for the determination of these compounds. However, analyte extraction has to be further optimized, in particular for HAcAms, in oder to improve analyte recoveries (especially for the trihalogenated species that were not recovered with the applied analytical methodologies). Method sensitivities allowed the quantification of the monohalogenated and dihalogenated species in the ng/L range. Moreover, the stability of the analytes in the extracts and in the standard solutions should be further investigated, in order to ensure reliability of the results. All monohalogenated and dihalogenated DBPs were found in the investigated samples. The formation of iodo-DBPs was enhanced during chloramination scenarios. However, there is not a direct relation between iodo-DBP formation and increasing amounts of seawater in the source waters. On the contrary, the seawater content of the tested waters had an effect on the speciation of bromine containing and chlorine containing HAAs, by increasing brominated species at increasing seawater amounts.