The Tris(hydroxymethyl)aminomethane (TRIS) salt of a substituted 5,6,7,8-tetrahydro-1,8-naphthyridine compound (I) in a mannitol-based formulation was stressed at various conditions. Liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) analyses of the stressed samples revealed that oxidation and dimerization were the primary degradation pathways for this compound. 1H- and 13C-nuclear magnetic resonance (NMR) spectroscopy were used to characterize the isolated dimers. The aromatized degradate, N-oxide, amide, and three dimeric products were all confirmed by either LC/MS using authentic standards or NMR spectroscopy. In general, the aromatized product was always the primary degradate produced under all stress conditions. When stressed at 80 degrees C, the TRIS counterion also underwent thermal degradation to yield formaldehyde in situ which reacted with the parent compound to form a unique methylene-bridged dimeric product and an N-formyl degradate. A minor condensation product between the compound I and the TRIS counterion was also detected in the 80 degrees C stressed samples. Under 40 degrees C/75% RH stress conditions, TRIS derived degradates were insignificant, while dimers formed by compound I became predominant. In addition, two hydroxylated products (7-OH and 5-OH) were also detected. Mechanisms for the formation of the oxidative and dimeric degradates were proposed.