N-Nitrosamines and nitrosamides can initiate cancer. These studies evaluated the stability and reactivity of 2-nitrosoamino-3-methylimidazo[4,5-f]quinoline (N-NO-IQ) to assess its possible role in the initiation of colon cancer by 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). 14C-N-NO-IQ was incubated with different solvents and pHs in the presence and in the absence of nucleophiles and analyzed by HPLC. The products identified by electrospray ionization mass spectrometry include 2-chloro-3-methylimidazo[4,5-f]quinoline (2-Cl-IQ), 2,2′-azo-3,3′-dimethylimidazo[4,5-f]quinoline (AZO-IQ), 2-azido-IQ (2-N3-IQ), 3-methylimidazo[4,5-f]quinoline (deamino-IQ), and IQ. A variety of organic solvents were tested with 0.1 N HCl. 2-Cl-IQ and IQ were formed following acidification of all solvents. AZO-IQ was only formed in methanol. Deamino-IQ was the major product formed in all of the alcohols tested, except for methanol. Under acidic conditions that completely convert N-NO-IQ in 5 min (acetonitrile with 0.1 N HCl), 62% of N-NO-IQ remains after 30 min if dimethyl sulfoxide is substituted for acetonitrile. N-NO-IQ was stable in the physiologic pH range of 5.5-9.0 and did not react with nucleophiles over a 4 h period at pH 7.4 and 37 °C. At acidic pH (pH ≤2.0) for 30 min and 37 °C, N-NO-IQ becomes labile forming electrophile(s), which combine with biologically relevant nucleophiles. The reaction of N-NO-IQ at pH 2.0 followed first-order kinetics (t1/2 = 10 ± 2 min) and was significantly increased in 10 mM NaN3 (t1/2 = 2 ± 0.1 min). 2-N3-IQ was the major product observed in the latter incubation. N-NO-IQ binding to DNA at pH 2.0 is 100-fold more than that at pH 7.4. At pH 2.0, greater than 90% of the binding was inhibited by 10 mM NaN 3. Thus, N-NO-IQ forms a reactive electrophile(s) at acidic pH, which binds DNA. N-NO-IQ reaction products may depend on the pH and the hydrophobic milieu of cells or tissues.