Hydrothermal-controlled selective cleavage of C-beta-O and C-alpha-C-beta bonds was well realized via modification the side-chain structure of lignin. To elucidate the reaction pathway and the origin of selectivity, the sites of the benzylic alcohol (C-alpha-OH) and benzylic ketone (C-alpha=0) on the cleavage mechanisms of beta-O-4 lignin dimers were investigated. Results clearly demonstrated the hydrothermolysis process could be divided into three stages: (1) intramolecular elimination reactions, (2) the cleavage of C-alpha-C-beta or C-beta-O bonds and (3) secondary reactions of intermediates. Reaction pathways were highly susceptible to the side-chain structure. The C-alpha-C-beta and C-beta-O bonds of 1-(4-hydroxy-3-methoxyphenyI)-2-(2-methoxyphenoxy)-ethanone tended to directly cleave and form aromatic monomers. However, many different species dimers were produced via the intramolecular elimination reactions when the hydroxyl group was introduced at the C-alpha or C-gamma position, which participated in a variety of reaction mechanisms, resulting in more complex products distribution. In addition, the results also indicated that dehydration of C-alpha-OH group significantly facilitated the cleavage of C-beta-O bonds, but oxidation of the Ca-OH into C-alpha = 0 group weakened the C-alpha-C-beta bond. Based on the compositions and structures of products, several cleavage pathways of dimers were proposed, explaining well the origin of selectivity. (C) 2017 Elsevier B.V. All rights reserved.