This document specifies a collection of methods for the characterization of extracts obtained from agro-industrial residues and by-products, with the purpose to be evaluated for being recycled/ upcycled as antioxidant additives in food, cosmetics, and/ or nutraceutical formulations. The collection of methods applies to wastes and by-products from vegetables and fruits processing. A method for the determination of total polyphenol content by a colorimetric assay using Folin-Ciocalteu phenol reagent [1] is provided. Along with polyphenols, this method quantifies flavonoids, ascorbic acid, reducing sugars, some amino acids, and some aromatic amines. For the determination of total flavonoids content, a method based on the colorimetric assay using sodium borohydride/ Chloranil based reagent [2] is provided. Amongst flavonoids are included: flavones, flavonols, flavonones, falvonols, isoflavonoids, flavanols, and anthocyanins. For the determination of the antioxidant capacity of extracts, three different methods based on a colorimetric assay using 2,2' -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) [3], 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and ferric-tripyridyltriazine (Fe3+ - TPTZ) [4], respectively, are specified.