The paper presents the results of experimental investigations on hydrogen absorption and desorption in the metal hydride reactor RS-1 containing 81 kg of La0.5Nd0.5-Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound at 60 degrees C isothermal conditions. During the reactor-scale measurements of pressure-composition isotherms we observe pressure artifacts; pressures for absorption are higher and for desorption are lower than equilibrium pressures obtained after cooling down and reheating of the reactor. This thermal relaxation procedure removes the pressure artifacts. The observed effect is similar to the large aliquot effect, and mostly affects the desorption isotherm. The highest measured pressure difference is 0.7 bar (11%). (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.