Naturally occurring hydroxycinnamic acids, such as ferulic and sinapic acids, are known to possess antioxidant activity. In this study, ferulic acid and sinapic acid were covalently attached to oat spelt arabinoxylan and birch wood glucuronoxylan by esterification in a two-step feasible synthesis to generate modified xylans with various degrees of substitution. The obtained derivatives were fully analyzed by FT-IR, NMR, and HPSEC experiments to confirm the esterification of xylans and the degree of substitution. The antioxidative potential of the conjugates was evaluated using the emulsion lipid oxidation test. The results demonstrate that the derivatized xylans inhibited lipid oxidation notably better than the native oat spelt and birch wood xylans. It was found that ferulic acid esters of glucuronoxylan were more efficient antioxidants than those of arabinoxylan and that sinapic acid xylan esters were more efficient than their ferulic acid counterparts.