Chemiluminescence of 3-methylindole (3-MIH) based on electrogeneration of superoxide ion (O-2 (*)) in it air-saturated acetonitrile solutions containing no base was investigated by the use of spectroelectrochemical techniques for the first time, The emission spectrum was observed around -0.2 V versus Ag \ AgCl \ NaCl(sat) after the electrogeneration and the subsequent reoxidation of O-2(- *) (formal potential. E-O2/O2(0) . = -0.86 V) on the electrode surface. This spectrum revealed maximum intensity around 475 nm. On the cyclic voltammogram obtained together with the chemiluminescence at the 475-nm curve, an oxidation peak apart from the reoxidation peak of O-2(- *) was observed at about -0.2 V. This oxidation process around -0.2 V had a close connection with the chemiluminescence. and was considered to be due to the electrooxidation of 3-MI (i.e. a conjugate base of 3-MIH) to 3-MI* (i.e. a one-electron oxidation product, a radical species of 3-MI-). The electrogenerated O-2 (.) could function as a proton acceptor to 3-MIH. This reaction generates 3-MI- and hydroperoxyl radical (HO2.), In the chemiluminescence observed around -0.2 V, the radical--radical coupling reaction between 3-MI. and HO2. was considered to result in the formation of a 1,2-dioxetane-like intermediate to lead to the emission of light. The chemiluminescent reaction mechanism of 3-MIH was discussed in detail in comparison with that of the MCLA (a Cypridina luciferin analogue)-O-2 (.) system.