Formation and speciation of trihalomethanes (THMs) in chlorinated groundwaters with low total organic carbon (TOC), bromide, and specific ultraviolet absorbance (SUVA(254nm)) levels were investigated. Groundwaters from three wells and 23 different points in the distribution system were monitored and sampled for a duration of 15 months. On-line and off-line monitoring of the distribution system indicated the integrity of the distribution system without any sort of contamination. Chlorine demand of the groundwaters was very low due to low levels of TOC, particulate matter and reduced inorganic species. Average TOC and turbidity levels of the three wells during the sampling period were 0.34 mg/L and 0.23 NTU, respectively. SUVA(254nm) values of groundwaters were generally <2.0 L/mg-m indicating that natural organic matter (NOM) in these waters is of dominantly non-humic and hydrophilic character. For all the sampling points, average THM concentrations of the 15-month sampling period were always <= 20 mu g/L. The formation of THMs could not be correlated to TOC, UV(254nm) absorbance, SUVA(254nm), or chlorine dosages. Some special NOM moieties, which cannot be captured by these NOM parameters and are most probably in non-humic, non-UV(254nm) absorbing and/or hydrophilic nature, appeared to be responsible for THM formations in these groundwaters. Most of the NOM in these waters was non-UV(254nm) absorbing fractions. The formation of chlorinated THM species was more favorable compared with brominated species, mainly due to much lower bromide concentrations than those of chlorine.