Betalains are water-soluble pigments with high antiradical capacity which bestow bright colors on flowers and fruits of most plants of the order Caryophyllales. They are classified as betacyanins, exhibiting a violet coloration, and betaxanthins, which exhibit yellow coloration. Traditionally, betalains have been defined as condensation products of betalamic acid with different amines and amino acids, but the implication of the pigment structure for their properties has not been investigated. This paper explores different structural features of the betalains, revealing the clues for the switch from yellow to violet color, and the loss of fluorescence. A relevant series of 15 betalain-related compounds (both natural and novel semisynthetic ones) is obtained and characterized by chromatography, UV-vis spectrophotometry, fluorescence, and electrospray ionization mass spectroscopy. Antiradical properties of individual pure compounds in a broad pH range are studied under the ABTS(*+) radical assay. Relevance of specific bonds is studied, and differences between betaxanthins and betacyanins are used to explore in depth the structure-antiradical activity relationships in betalains.