Cyanogen chloride (CNCl) is a disinfection byproduct found in chlorinated and chloraminated drinking water. Although there is an apparent greater association of CNCl with chloraminated water relative to chlorination systems, it was not clear whether these phenomenological observations are explained by differences in the stability or formation potentials of CNCl between the two disinfectants. In this study, the stability of CNCl was examined in the presence of free chlorine and monochloramine using membrane introduction mass spectrometry. CNCl decomposes relatively rapidly when free chlorine is present but is stable in the presence of monochloramine. The decomposition kinetics and observed reaction products are consistent with a hypochlorite-catalyzed hydrolysis mechanism, and the rate law is described by (d[CNCl]/dt) = - k OCl[CNCl][OCl-]. At 25°C, pH 7, and a free chlorine residual of 0.5 mg/L as Cl2, the half-life of CNCl is approximately 60 min, suggesting significant decomposition is expected over disinfection time scales. Under some winter season temperature conditions, however, the decay half-life of CNCl can be longer than typical disinfection contact times. The results of this study demonstrate that the observed association of CNCl with chloramination systems can in part be explained by the differences in its stability with chlorine and chloramines.