Sulfite ion reacts with 2,4,6-trinitrobenzaldehyde in aqueous solution of ionic strength 0.14 M to yield a 1 :1 o-con~plex at low sulfite concentrations and a 2: 1 o-complex at high sulfite concentrations. The visible absorption characteristics of these two o-complexes as well as equilibrium constants and relevant thermodynamic parameters for their formation have been determined. P.n~.r. studies demonstrate that two isomeric forms of the 1 : 1 o-complex occur in aqueous dimethylsulfoxide solution and two isomeric forms of the 2 : l o-complex occur in water. Iri both the 1 :1 and the 2 : l complexes isomers are found which contain sulfite ion attached to the C-1 carbon and one of the isomeric 2 : 1 o-complexes is postulated to exist in cis and trans forms. Observations are presented concerning the kinetics of the very rapid formation of the 1 :1 o-complex in water.L'ion si~lfite reagit avec la trinitro-2,4,6 benzaldehyde en solution aqileilse de force ioniqi~e 0.14 M pour donner iln complexe I : 1 a des concentrations faibles de si~lfite et iln coniplexe 2: 1 i des concentrations Clevees de sulfite. On a determine les caracteristiqi~es d'absorption de ces deux complexes ainsi qile les constantes d'kquilibres et les parametres thermodynaniiques-se rapportant a leur formation. Des etudes en r.m.p. ont montrt que deux fornies isomeriques du complexe 1 : 1 se forment en solution aqueuse de sulfoxyde de dimethyle et que deux fornies isomeriques du complexe 2:l se forment dans I'eai~. On a trouvk dans les deux coniplexes I : 1 et 2: 1 des isomtres qui contiennent l'ion sulfite relie ail carbone C-I. De meme on admet que I'un des con~plexes 2: 1 isomeriques existe sous des formes cis et trans. On prksente aussi des observations se rattachant a la cinetique de formation tres rapide du complexe 1 :