Abstract We investigated the reaction of Guo with nitrous acid in the presence of NaCl. When 1 mM Guo was incubated with 100 mM NaNO 2 and 2 M NaCl in sodium acetate buffer at pH 3.2 and 37 °C, 2-chloroinosine (2-Cl-Ino) was produced in addition to oxanosine (Oxo) and xanthosine (Xao). The yield of 2-Cl-Ino was 0.033 mM at an incubation time of 2 h. Under the same reaction conditions, GMP and dGuo gave rise to the corresponding 2-chloro derivatives with comparable yields. All the 2-chloro derivatives were fairly stable ( t 1 2 >360 h) at physiological pH and temperature. To elucidate the reaction mechanism of the chlorination, the diazoate derivative of Guo, a reaction intermediate of the Guo–HNO 2 system, was employed as a starting compound. When the diazoate was incubated with 2 M NaCl in a neutral solution, 2-Cl-Ino was produced in addition to Oxo and Xao. These results suggest that the 2-chloro derivatives can be produced from foodstuffs in the human stomach and may have potential importance as a carcinogen causing gastric cancer.