Betalains are one of the most common groups of plant pigments found in nature, especially in red beetroot (Beta vulgaris L.) which is the main commercially exploited source of betalains produced in the form of concentrates or powders. This report presents results of thermal decarboxylation studies on betacyanins present in a specifically purified highly concentrated betalain-rich extract (BRE). The first tentative structures formed by decarboxylation of the main pigment present in BRE, betanin and its diastereomer, were established by means of liquid chromatography coupled to diode array detection and electrospray ionization tandem mass spectrometry (LC-DAD-ESI-MS/MS). In the extract, two new isomeric bidecarboxylated betanins were tentatively identified. A high rate of generation of 2-decarboxy-betanin/-isobetanin which are present in the BRE extract at very low level was observed, which was dependent on the starting concentration of the BRE substrate. The bidecarboxylated derivatives were generated at a higher rate mostly from 17-decarboxy-betanin/-isobetanin as well as 15-decarboxy-betanin by further decarboxylation at carbon C-2. Further studies will be performed to demonstrate if the decarboxylated betanins being products of heating B. vulgaris preparations can be used for various food applications with new health-promoting actions and colorant properties.