This study investigates the effect of bromide ions and surrogate water quality factors on the possible formation of halogenated trihalomethanes (THMs) in Thailand's water treatment facilities. Bromide ions, which could not be well removed via conventional treatment processes, combine with dissolved organic matter (DOM) during chlorination, forming brominated THMs such as bromodichloromethane (BDCM) and dibromochloromethane (DBCM). Seasonal fluctuations in water quality, including dissolved organic carbon (DOC), specific ultraviolet absorbance (SUVA), and DOM molecular weight distribution, were investigated for their impact on THMs formation. The results found that the average bromide contents in raw surface water in rainy winter and summer were 9.77, 17.70, and 43.60 µg/L, respectively. To evaluate the effect of bromide on brominated THMs (Br-THMs) formation, the bromide-to-chlorine ratio (Br-/Cl2) was used. An increase in the Br-/Cl2 ratio led to a rise BDCM and DBCM concentrations, from 25.6 and 2.3 µg/L to 57.0 and 4.5 µg/L, respectively. This is because of the reactivity of hypobromous acid (HOBr) and its dissociated form, hypobromite anion (OBr-). In contrast, the decrease of trichloromethane (TCM) was reduced from 440 to 168.2 µg/L when Br-/Cl2 ratio increased. The Bromine Incorporation Factor (BIF) has been used for studies to determine the extent of bromine substitution during THM formation. The BIF is higher in the summer, suggesting more bromine substitution in THM formation during that season. The results of surrogate parameters including DOM fractions and the Excitation-Emission Matrix (EEM) fluorescence spectroscopy, were also investigated in this study to assess their impact on Br-THMs formation. DOM fractions exhibited molecular weights of 7.0-27.3 kDa in different seasons, resulting in Br-THMFP. The EEM fluorescence spectroscopy results revealed that humic substances, particularly humic acids and humic-like substances, are dominant in water samples, considerably increasing disinfection byproducts (DBPs) production potential. The highlight of these studies was the identification of regions or treatment practices associated with higher Br-THMs formation, which can inform Thai public health policies and guidelines while contributing to the development of more stringent water quality regulations tailored to the country's needs.