Hydrothermal carbonization of soft rush (Juncus effusus) has been studied in the temperature range of 453-513 K. Citric acid at the concentration of 0.2 kg m(-3) did not have a significant catalytic effect. A kinetic model is suggested for substrate hydrolysis and the formation of hydrochar as well as byproducts remaining dissolved. Specifically, increasing hydrochar yield at higher substrate concentration can be described with a reaction order of 1.53 for hydrochar formation from intermediate products. (C) 2015 Elsevier Ltd. All rights reserved.