Anthocyanins are plant-derived pigments with diverse applications due to their favourable spectral and biological properties, which are often modulated through enzymatic addition of structural modifications. However, evidence indicates that anthocyanin glycosyl groups may be modified by organic acids in solution through an acid-catalysed esterification. Here, the anthocyanin standard cyanidin-3-glucoside is shown to undergo esterification in aqueous conditions with citric acid, acetic acid, and most efficiently, formic acid. Extended to a biological sample, red cabbage anthocyanins incubated with formic acid resulted in 53 unique species from 9 original anthocyanins. From these findings a key structural determinant to predict formylation patterns is identified: the precise glucosyl unit where hydroxycinnamic acid acylation occurs, which dictates the number of free primary alcohol groups in the anthocyanin available for formylation. This study demonstrates the unexpected structural complexity these reactions introduce into samples that necessitate careful consideration in anthocyanin handling. Harnessing these reactions in future may help produce anthocyanins with controlled structures and properties for downstream applications.