The decomposition of amitriptyline hydrochloride upon autoclaving in a buffered solution (pH 6.8) was investigated. Three major decomposition products [3-(propa-1,3-dienyl)-1,2:4,5-dibenzocyclohepta-1,4-diene, dibenzosuberone, and 3-(2-oxoethylidene)-1,2:4,5-dibenzocyclohepta-1,4-diene] were detected and identified by chromatographic and spectroscopic techniques. Evidence is presented that the latter two compounds are formed by further oxidation of 3-(propa-1,3-dienyl)-1,2:4,5-dibenzocyclohepta-1,4-diene, and a possible decomposition pathway is outlined.