Abstract Betanin, phyllocactin (malonylbetanin) and hylocerenin (3‐hydroxy‐3‐methylglutarylbetanin) were isolated from purple pitaya ( Hylocereus polyrhizus [Weber] Britton & Rose) juice, and their degradation products generated by heating at 85°C were subsequently monitored by high‐performance liquid chromatography/electrospray ionization tandem mass spectrometry. Thermal degradation of phyllocactin and hylocerenin in purified solution excluding the alleged protective effects by the juice matrix is reported for the first time. Betanin was predominantly degraded by hydrolytic cleavage, while decarboxylation and dehydrogenation were of minor relevance. In contrast, hylocerenin showed a strong tendency to decarboxylation and dehydrogenation, hydrolytic cleavage of the aldimine bond occurring secondarily. Phyllocactin degradation was most complex because of additional decarboxylation of the malonic acid moiety as well as generation and subsequent degradation of betanin due to phyllocactin demalonylation. Upon prolonged heating, all betacyanins under observation formed degradation products characterized by an additional double bond at C 2 C 3 . Hydrolytic cleavage of the aldimine bond of phyllocactin and hylocerenin yielded previously unknown acylated cyclo ‐dopa derivatives traceable by positive ionization, while application of ESI(–) facilitated the detection of a glycosylated aminopropanal derivative and dopamine, which have never been described before as betanin degradation products. Copyright © 2005 John Wiley & Sons, Ltd.