The reactivity of 1-methylbenzimidazole 3-oxide (I), which is a five-membered heteroaromatic N-oxide, was investigated. Deoxygenation with phosphorus trichloride, reactions with acetic anhydride and some anionide reagents, and the Reissert reaction were examined. Normal products were obtained in all cases, a result which can be expected from the reactions of the six-membered N-oxide. I reacted with methyl cyanoacetate to give methyl α-cyano-1-methyl-2-benzimidazoleacetate. I and 1-methoxybenzimidazole reacted with methyl iodide to give 1-methyl-3-methoxybenzimidazolium iodide in both cases. In solution, I is liable to rearrange to 1-methyl-2-benzimidazolinone and pyrolysis of I gave 1, 1'-dimethyl-2, 2'-bibenzimidazole 3-oxide and 1, 1'-dimethyl-2, 2'-bibenzimidazole.