Studies were conducted to investigate the hypothesis that N-nitrosodimethylamine (NDMA) can be produced from the reaction involving monochloramine. Dimethylamine (DMA) was used as a potential precursor. NDMA was formed from the reaction between DMA and monochloramine. The formation of NDMA increased with increased monochloramine concentration. No significant amount of NDMA was formed in Iowa River water. However, addition of 0.5 mM and 1.0 mM of monochloramine to Iowa River water produced 5.6 ng/L and 14.1 ng/L, respectively. The mass spectra of the NDMA formed from DMA and 15 N isotope labeled monochloramine , 15 NH2Cl, showed that the source of the nitro group nitrogen in NDMA was from monochloramine. NDMA, therefore, could possibly be a "new" disinfection by-product formed in mechanism that may not involve N-nitrosation pathway. The reaction mechanism of NDMA formation during chloramination or chlorination in the presence of ammonia was proposed. Includes 14 references.